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>

src/web/api/session.ts

@@ -65,6 +65,13 @@ export function createSessionRouter(
       res.status(401).json({ error: "unauthenticated" });
       return;
     }
+    // A guest session is only valid while guest mode is enabled. Disabling it
+    // immediately invalidates any in-flight guest sessions (mirrors createRequireAuth).
+    if (result.role === "guest" && !getGuestConfig().enabled) {
+      clearSessionCookie(res);
+      res.status(401).json({ error: "unauthenticated" });
+      return;
+    }
     req.user = { id: result.userId, username: result.username, role: result.role };
     const token = extractSessionToken(req.headers.cookie);
     if (token) setSessionCookie(res, token);