test(frontend): harness expansion + utility coverage push (F0 + F6) (#539)

* test(frontend): expand harness for store/component/contract tests

- factories: makeSubsonicSong, makeServer, makeAuthState, makeQueueState
- storeReset.ts: per-test reset for player/auth/preview/orbit stores
- mocks/subsonic.ts: realistic fixtures + stream/cover URL helpers
- mocks/browser.ts: ResizeObserver/IntersectionObserver/matchMedia/clipboard/object URLs
- mocks/tauri.ts: tauriMockListenerCount for listener-lifecycle regression tests
- renderWithProviders: pin i18n language to 'en' by default; { language } opt-out
- vitest.config: pool 'forks' + isolate to avoid module-mock + Zustand-global flake
- README: documented patterns, store-reset policy, i18n rule, isolation rationale

* test(frontend): bump utility coverage + expand hot-path gate

serverMagicString: 71→100% (encode/decode rejection branches, clipboard
fallback paths). shareLink: 69→97% (all entity kinds, queue trim, orbit
decoder, findServerIdForShareUrl). dynamicColors: 44→100% (extractCoverColors
DOM paths via Image / canvas / fetch mocks).

Gate adds shareLink.ts and dynamicColors.ts — both stable above 95%.
Comments updated for the new floor and the M4 hard-gate handoff.

src/test/mocks/tauri.ts

@@ -69,6 +69,17 @@ export function emitTauriEvent(event: string, payload: unknown): void {
   for (const cb of eventListeners.get(event) ?? []) cb(payload);
 }
 
+/**
+ * How many `listen()` callbacks are currently registered for `<event>`.
+ *
+ * Use for regression tests of listener lifecycle — e.g. re-initializing a
+ * store should not double-register `audio:progress` handlers. See
+ * `feedback_global_shortcut_double_fire` for the canonical motivating bug.
+ */
+export function tauriMockListenerCount(event: string): number {
+  return eventListeners.get(event)?.length ?? 0;
+}
+
 /** Clear all handlers + listeners + call counts. Wired to `beforeEach` below. */
 export function resetTauriMocks(): void {
   invokeHandlers.clear();