test(ui): MiniPlayer + FullscreenPlayer (§4.5 regression) (Phase F5c) (#548)

MiniPlayer.test.tsx (4): mounts without throwing, renders the
always-present titlebar controls (Pin + Open main window — Close is
Linux-only and lives on the manual smoke list per pick 4a), click on
Open main window / Close does not throw, clicking the Pin button
flips the alwaysOnTop label between "Unpin" and "Pin on top". Bridge
contract (mini:ready / mini:sync, geometry persistence) deferred to
B-tier phase B5 -- jsdom does not model two webviews.

FullscreenPlayer.test.tsx (9): renders the labelled Fullscreen Player
dialog + Close Fullscreen control. Control wiring: Close calls
onClose, Stop calls stop, Previous calls previous, Next calls next,
Repeat cycles via toggleRepeat.

§4.5 of the v2 plan -- useCachedUrl(coverUrl, coverKey, false)
regression. Mocks the CachedImage module so the call args are
observable. Pins:
  - the 500 px cover-art call passes opt=false (no fetchUrl fallback;
    prevents double crossfade fetchUrl -> blobUrl);
  - the 300 px art-box call passes opt=true (default behaviour).
A refactor that "tidies up" the useCachedUrl call sites would silently
regress the FS player cover; this test makes it loud.

Harness fix: vi.mock for @tauri-apps/api/event now returns an async
emit that resolves -- components chain .catch() on emit which crashed
on the bare vi.fn() return value during first-render useEffect.
Benefits any future component test that mounts something using emit.

Frontend suite: 399 -> 412 tests (+13). F5 (and Phases F0-F6) complete.

src/test/setup.ts

@@ -89,8 +89,11 @@ vi.mock('@tauri-apps/api/core', () => ({
 
 vi.mock('@tauri-apps/api/event', () => ({
   listen: vi.fn(),
-  emit: vi.fn(),
-  once: vi.fn(),
+  // Components chain `.catch()` on emit; return a resolved Promise so the
+  // chain doesn't throw "Cannot read properties of undefined (reading
+  // 'catch')" inside a useEffect on first render.
+  emit: vi.fn(async () => undefined),
+  once: vi.fn(async () => () => {}),
 }));
 
 // Linker for Tauri shell / dialog / store plugins — same idea. Extend as needed.