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/routes/photos/+page.svelte

@@ -81,6 +81,8 @@
 
 	const groups = $derived.by(() => {
 		const out: Array<{ key: string; label: string; photos: PhotoItem[] }> = [];
+		// Transient scratch map built inside $derived.by and discarded — not reactive state.
+		// eslint-disable-next-line svelte/prefer-svelte-reactivity
 		const index = new Map<string, number>();
 		for (const p of items) {
 			const d = new Date(photoTimestamp(p));