perf(common): stop useTimeAgo re-rendering every second, add re-render profiler

measure-rerenders.mts counts React commits on an untouched board and attributes each
to the shallowest component that re-rendered, i.e. where the update entered the tree.

What it found: an idle board commits 73-96 times per minute, re-rendering
@mantine/core/Box over 30,000 times in 60s. That is the churn behind the 650 MB
browser peak — transient allocation rather than retention. About 80% of those commits
enter at the same anonymous depth-2 provider, props [value, children], value {appDir}:
Next.js's internal HeadManagerContext, whose value is a fresh object literal on every
App Router render, so each render re-renders the whole app beneath it. The rest enter
at __next_metadata_boundary__ / <title>, pointing the same way.

The trigger is inside Next's router/metadata handling — application code has no
polling router.refresh() — so nothing here fixes it. It is the next thing to chase and
it dominates everything downstream of it.

useTimeAgo called setState on a 1s interval even when the label was unchanged ("3
minutes ago" holds for a full minute), re-rendering its subtree ~60x/minute per
consumer for an identical string. It now compares before setting. Recorded as a
correctness fix and NOT a measured win: idle commits stayed inside their 73-96 range
afterwards, because the app-root churn dominates.

pnpm test 1794/1795 (pre-existing pi-hole Testcontainers flake); typecheck clean
across 37 packages; no new lint errors.

scripts/benchmarks/measure-rerenders.mts

@@ -0,0 +1,175 @@
+/**
+ * Counts React commits and per-component re-renders on an idle board.
+ *
+ * The 650 MB browser heap was mostly transient allocation, i.e. re-render churn rather
+ * than retention, so the number that matters is how often components re-render while
+ * nobody is touching the page. Hooks into the devtools commit callback and attributes
+ * each commit to the components that actually re-rendered in it.
+ *
+ *   STRESS_BASE_URL=http://127.0.0.1:PORT \
+ *   node --experimental-strip-types scripts/benchmarks/measure-rerenders.mts
+ */
+import { chromium } from "@playwright/test";
+
+const baseUrl = process.env.STRESS_BASE_URL;
+if (!baseUrl) throw new Error("STRESS_BASE_URL is required");
+const username = process.env.STRESS_USERNAME ?? "demo";
+const password = process.env.STRESS_PASSWORD ?? "demodemo";
+const boardName = process.env.STRESS_BOARD ?? "default";
+const observeMs = Number(process.env.RERENDER_OBSERVE_MS ?? 60_000);
+
+const browser = await chromium.launch();
+const context = await browser.newContext({ viewport: { width: 1920, height: 1080 } });
+
+await context.addInitScript(() => {
+  const state = { commits: 0, byComponent: new Map<string, number>(), roots: new Map<string, number>(), started: 0 };
+  (globalThis as unknown as Record<string, unknown>).__homarrRenderStats = state;
+
+  const nameOf = (fiber: { type?: unknown; elementType?: unknown }) => {
+    const type = (fiber.type ?? fiber.elementType) as
+      | string
+      | { displayName?: string; name?: string }
+      | null
+      | undefined;
+    if (typeof type === "string") return `<${type}>`;
+    if (typeof type === "function" || (type && typeof type === "object")) {
+      return (
+        (type as { displayName?: string; name?: string }).displayName ??
+        (type as { name?: string }).name ??
+        "(anonymous)"
+      );
+    }
+    return null;
+  };
+
+  (globalThis as unknown as Record<string, unknown>).__REACT_DEVTOOLS_GLOBAL_HOOK__ = {
+    renderers: new Map(),
+    supportsFiber: true,
+    inject: () => 1,
+    onCommitFiberUnmount: () => undefined,
+    onPostCommitFiberRoot: () => undefined,
+    onCommitFiberRoot: (_id: unknown, root: { current?: unknown }) => {
+      if (!state.started) return;
+      state.commits++;
+      // Walk only fibers React flagged as updated in this commit.
+      const seen = new Set<unknown>();
+      // Shallowest re-rendering component in this commit — i.e. where the update
+      // entered the tree. Everything below it re-rendered as a consequence.
+      let shallowest: { name: string; depth: number; fiber: Record<string, unknown> } | null = null;
+      const walk = (fiber: Record<string, unknown> | null | undefined, depth: number) => {
+        if (!fiber || seen.has(fiber) || depth > 2000) return;
+        seen.add(fiber);
+        const alternate = fiber.alternate as Record<string, unknown> | null;
+        // A fiber re-rendered if it committed new work over a previous version.
+        if (alternate && fiber.actualDuration !== undefined && (fiber.actualDuration as number) > 0) {
+          const name = nameOf(fiber as never);
+          if (name) {
+            state.byComponent.set(name, (state.byComponent.get(name) ?? 0) + 1);
+            if (!shallowest || depth < shallowest.depth) shallowest = { name, depth, fiber };
+          }
+        }
+        walk(fiber.child as never, depth + 1);
+        walk(fiber.sibling as never, depth);
+      };
+      walk((root.current ?? null) as never, 0);
+      if (shallowest) {
+        const found = shallowest as { name: string; depth: number; fiber?: Record<string, unknown> };
+        // Anonymous components are common; the ancestor chain identifies them.
+        const chain: string[] = [];
+        let up = found.fiber?.return as Record<string, unknown> | undefined;
+        for (let i = 0; i < 8 && up; i++, up = up.return as Record<string, unknown> | undefined) {
+          const name = nameOf(up as never);
+          if (name) chain.push(name);
+        }
+        // At the very top the ancestor chain is empty, so describe it by what it
+        // renders and which props it takes instead.
+        const descendants: string[] = [];
+        let down = found.fiber?.child as Record<string, unknown> | undefined;
+        for (let i = 0; i < 6 && down; i++, down = down.child as Record<string, unknown> | undefined) {
+          const name = nameOf(down as never);
+          if (name) descendants.push(name);
+        }
+        const props = Object.keys((found.fiber?.memoizedProps as object) ?? {}).slice(0, 8);
+        // For a Context.Provider the value's shape names the context far better
+        // than a minified component name does.
+        const rawValue = (found.fiber?.memoizedProps as { value?: unknown } | undefined)?.value;
+        const valueShape =
+          rawValue && typeof rawValue === "object"
+            ? `value{${Object.keys(rawValue).slice(0, 10).join(",")}}`
+            : `value:${typeof rawValue}`;
+        const key =
+          `${found.name} @depth${found.depth}` +
+          ` props[${props.join(",")}] ${valueShape}` +
+          ` > ${descendants.join(" > ")}` +
+          (chain.length ? ` < ${chain.join(" < ")}` : "");
+        state.roots.set(key, (state.roots.get(key) ?? 0) + 1);
+      }
+    },
+    __start: () => {
+      state.started = Date.now();
+      state.commits = 0;
+      state.byComponent.clear();
+      state.roots.clear();
+    },
+  };
+});
+
+const page = await context.newPage();
+await page.goto(`${baseUrl}/auth/login`, { waitUntil: "networkidle" });
+await page.getByLabel("Username").fill(username);
+await page.locator("#password").fill(password);
+await page.locator("css=button[type='submit']").click();
+await page.waitForFunction(() => !window.location.pathname.includes("/auth/login"), undefined, { timeout: 60_000 });
+
+await page.goto(`${baseUrl}/boards/${boardName}`, { waitUntil: "domcontentloaded" });
+await page.locator("[data-homarr-dev-benchmark-board]").waitFor({ state: "visible", timeout: 120_000 });
+await page
+  .waitForFunction(
+    () => {
+      const items = [...document.querySelectorAll('[data-type="item"]')];
+      return (
+        items.length > 0 &&
+        items.every((item) => item.querySelector("[data-homarr-widget-ready],[data-homarr-widget-error]"))
+      );
+    },
+    undefined,
+    { timeout: 120_000 },
+  )
+  .catch(() => console.log("warn: widgets did not all settle"));
+
+// Let the initial render storm finish before counting steady-state churn.
+await page.waitForTimeout(5_000);
+await page.evaluate(() => {
+  (globalThis as unknown as Record<string, { __start: () => void }>).__REACT_DEVTOOLS_GLOBAL_HOOK__.__start();
+});
+
+console.log(`observing an idle board for ${observeMs / 1000}s (no interaction)...`);
+await page.waitForTimeout(observeMs);
+
+const stats = await page.evaluate(() => {
+  const state = (
+    globalThis as unknown as Record<string, { commits: number; byComponent: Map<string, number>; started: number }>
+  ).__homarrRenderStats as unknown as {
+    commits: number;
+    byComponent: Map<string, number>;
+    roots: Map<string, number>;
+    started: number;
+  };
+  return {
+    commits: state.commits,
+    elapsedMs: Date.now() - state.started,
+    top: [...state.byComponent.entries()].sort((a, b) => b[1] - a[1]).slice(0, 20),
+    roots: [...state.roots.entries()].sort((a, b) => b[1] - a[1]).slice(0, 12),
+  };
+});
+
+const perMinute = (stats.commits / stats.elapsedMs) * 60_000;
+console.log(
+  `\ncommits while idle: ${stats.commits} over ${(stats.elapsedMs / 1000).toFixed(0)}s (${perMinute.toFixed(1)}/min)`,
+);
+console.log("\nupdate entered the tree at (commit roots):");
+for (const [name, count] of stats.roots) console.log(String(count).padStart(8), ` ${name}`);
+console.log("\nre-renders by component:");
+for (const [name, count] of stats.top) console.log(String(count).padStart(8), ` ${name}`);
+console.log(`\n${JSON.stringify({ commits: stats.commits, perMinute: Number(perMinute.toFixed(1)) })}`);
+await browser.close();