fix(guest): deny favorites + auth-status reads to guests; UI polish

Consolidated fix wave from the final whole-branch review of guest mode.

- FIX 1 (critical): gate /api/favorites mount with requireNotGuest — the
  router keys off req.user.id (shared __guest__ principal), so guests could
  read/write a shared favorites bucket. Added focused guest-deny tests.
- FIX 2: gate GET /api/auth/status and /api/auth/qrcode/status with
  requireNotGuest so config reads no longer leak to guests.
- FIX 3: requireAuthInline in createSessionRouter now rejects guest sessions
  with 401 once guest mode is disabled (mirrors createRequireAuth), so /me
  stops returning guest data after an admin disables the feature.
- FIX 4: Login guest button now sits BELOW the card (auth-page flex-direction
  column + guest-btn width 360px) instead of beside it.
- FIX 5: mobile mini-player transport buttons in App.vue are now per-button
  gated for guests (prev/play/next/mode/volume), mirroring Player.vue.
- FIX 6: refreshed stale "gated on player.control" seek comments in Player.vue
  and relabeled the now-stale quality-GET test.

npm test: 354/354 pass. npm run build: tsc + vue-tsc + vite all green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/views/Login.vue

@@ -73,6 +73,7 @@ async function enterAsGuest() {
 .auth-page {
   min-height: 100vh;
   display: flex;
+  flex-direction: column;
   align-items: center;
   justify-content: center;
   background: var(--bg-primary);
@@ -100,7 +101,7 @@ async function enterAsGuest() {
 .auth-card button:disabled { opacity: 0.6; cursor: progress; }
 .auth-error { color: #e26a6a; font-size: 13px; margin: 0; }
 .guest-btn {
-  height: 38px; margin-top: 4px; border-radius: var(--radius-sm);
+  width: 360px; height: 38px; margin-top: 4px; border-radius: var(--radius-sm);
   background: transparent; color: var(--text-secondary);
   border: 1px solid var(--border-color); cursor: pointer;
 }