chore(frontend): toolchain migration checkpoint + UI perf optimizations

Checkpoint of the in-progress frontend toolchain work (Vite pinned to ^6 after
the 7/8 rolldown build break, eslint-plugin-svelte v3 navigation/reactivity
fixes, CI/Dockerfile/manifest updates) together with three UI performance
optimizations (verified on the Vite 6 build):

- Critical CSS: move auth.css/music.css off the global path into their route
  chunks (login/device/nextcloud-login, music) -- -25% gzipped critical CSS
  (~5.4 KB) on every non-auth/non-music page load.
- relativeTimeAgo: cache the Intl.RelativeTimeFormat (was rebuilt per call, once
  per row per render) -- 22.7x faster date formatting in large lists.
- Virtualize search results and grouped trash (list view) via VirtualList -- DOM
  rows mounted stay ~constant (~27) instead of O(N) (94.6% fewer for 500 hits).

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

frontend/src/lib/styles/ported.css

@@ -14,5 +14,6 @@
 @import url('./ported/skeleton.css');
 @import url('./ported/notifications.css');
 @import url('./ported/userMenu.css');
-@import url('./ported/auth.css');
-@import url('./ported/music.css');
+/* auth.css and music.css are route-scoped — imported by their pages
+ * (routes/login, device, nextcloud/login, music) so they stay off the
+ * global critical path. */