feat(http): set custom User-Agent on outbound integration requests (#6606)

* feat(http): set custom User-Agent on outbound integration requests

Set a default 'User-Agent: Homarr/<version> (+https://homarr.dev)' header
on all outbound HTTP requests made via fetchWithTrustedCertificatesAsync,
so Homarr's integration traffic is distinguishable in reverse-proxy,
WAF, and security-monitoring logs instead of appearing as generic undici.

Closes #6604

* refactor(version): centralize Homarr version in @homarr/version package

Replace fragile deep-relative 'package.json' imports across the monorepo
with a single source of truth: the new @homarr/version leaf package reads
the version once from the root package.json and exports it as VERSION.

Consumers now import by package name incl. the new User-Agent header in
fetchWithTrustedCertificatesAsync (packages/core):
  - core, api (info router), analytics, request-handler (update checker)
This also removes all manually-maintained relative version imports.

* fix(http): normalize User-Agent injection with Headers

Object-spreading fetchOptions.headers breaks when callers pass a Headers
instance (spread yields no entries) or a tuple array (numeric keys), and is
case-sensitive. Build a Headers instance and set the default User-Agent only
when no case-insensitive User-Agent entry exists, via a shared helper in both
fetch paths.

Adds unit tests covering record, Headers, tuple-array, and casing variants.

* revert(version): undo version refactor and tests, keep single-file change

Keep the User-Agent change as a single edit to
packages/core/src/infrastructure/http/request.ts. Remove the @homarr/version
package, the consumer migrations, and the unit tests.

packages/core/src/infrastructure/http/request.ts

@@ -14,6 +14,10 @@ import { UndiciHttpAgent } from "@homarr/core/infrastructure/http";
 import type { TrustedCertificateHostname } from "../certificates/hostnames";
 import { withTimeoutAsync } from "./timeout";
 
+import packageJson from "../../../../../package.json";
+
+export const getDefaultUserAgent = () => `Homarr/${packageJson.version} (+https://homarr.dev)`;
+
 export const createCustomCheckServerIdentity = (
   trustedHostnames: TrustedCertificateHostname[],
 ): typeof checkServerIdentity => {
@@ -79,6 +83,10 @@ export const fetchWithTrustedCertificatesAsync = async (
       async (signal) =>
         fetch(url, {
           ...fetchOptions,
+          headers: {
+            "User-Agent": getDefaultUserAgent(),
+            ...fetchOptions.headers,
+          },
           signal,
           dispatcher: agent,
         }),
@@ -89,6 +97,10 @@ export const fetchWithTrustedCertificatesAsync = async (
   const { bodyTimeout: _bodyTimeout, dispatcher: _dispatcher, ...fetchOptions } = options ?? {};
   return fetch(url, {
     ...fetchOptions,
+    headers: {
+      "User-Agent": getDefaultUserAgent(),
+      ...fetchOptions.headers,
+    },
     dispatcher: agent,
   });
 };