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/eslint.config.js

@@ -19,7 +19,10 @@ export default ts.config(
 		}
 	},
 	{
-		files: ['**/*.svelte'],
+		// `.svelte` components and `.svelte.ts`/`.svelte.js` rune modules are all
+		// parsed by svelte-eslint-parser under eslint-plugin-svelte v3; it needs the
+		// TS parser for the embedded/whole-file TypeScript or it chokes on type syntax.
+		files: ['**/*.svelte', '**/*.svelte.ts', '**/*.svelte.js'],
 		languageOptions: {
 			parserOptions: {
 				parser: ts.parser