refactor: drop server-seeded widget clock

The WidgetTimeProvider plumbed a server timestamp and the container's
timezone down to useWidgetNow so clock widgets would paint a real time in
the SSR HTML instead of "--:--". That traded a cosmetic first-paint win for
a wrong clock: the seeded timezone is the server's, so anyone whose timezone
differs from the container's saw an incorrect time for the whole hydration
window. Fixing that properly needs a client-set timezone cookie read during
RSC render, which is far more plumbing than the problem warrants.

Revert the clock to release/v2 behaviour ("--:--" until hydration) and drop
the provider, the @homarr/widgets/time export and their tests. The e2e clock
assertions now wait on hydration explicitly instead of the seeded paint.

Also restore the widget.beszel.getSystemStats persistence exclusion. #6289
added it deliberately to keep those large time-series payloads out of the
persisted cache, and this branch had removed it with no stated rationale.

Query-cache persistence and the background-refresh indicator are unchanged.

packages/api/src/test/query-cache.spec.ts

@@ -22,7 +22,7 @@ describe("isWidgetDataQueryKey", () => {
     [[["docker", "getContainers"], { type: "query" }], true],
     [[["integration", "byIds"], { type: "query" }], true],
     [[["widget", "app", "ping"], { type: "query", input: { id: "abc" } }], false],
-    [[["widget", "beszel", "getSystemStats"], { type: "query" }], true],
+    [[["widget", "beszel", "getSystemStats"], { type: "query" }], false],
     [[["widget", "beszel", "getSystems"], { type: "query" }], true],
     [[["app", "selectable"], { type: "query" }], false],
     [[["board", "getBoardByName"], { type: "query" }], false],
@@ -39,7 +39,7 @@ describe("isWidgetDataTrpcPath", () => {
     ["integration.byIds", true],
     ["docker.getContainers", true],
     ["widget.app.ping", false],
-    ["widget.beszel.getSystemStats", true],
+    ["widget.beszel.getSystemStats", false],
     ["board.getBoardByName", false],
   ])("classifies dashboard data path %s", (path, expected) => {
     expect(isWidgetDataTrpcPath(path)).toBe(expected);