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/config/drive/[uuid]/+page.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+	import { resolve } from '$app/paths';
 	import { page } from '$app/state';
 	import { onMount } from 'svelte';
 
@@ -70,7 +71,7 @@
 			<p class="muted">
 				{t('drive.not_found_body', "This drive doesn't exist or you don't have access to it.")}
 			</p>
-			<a class="link" href="/files">{t('drive.back_to_files', 'Back to Files')}</a>
+			<a class="link" href={resolve('/files')}>{t('drive.back_to_files', 'Back to Files')}</a>
 		</div>
 	{:else}
 		<h1>