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/components/DrivePicker.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import { goto } from '$app/navigation';
+	import { resolve } from '$app/paths';
 	import { page } from '$app/state';
 	import { onMount } from 'svelte';
 
@@ -55,7 +56,7 @@
 		} catch {
 			/* private mode / quota — silently fall back to default */
 		}
-		await goto(`/files/${d.root_folder_id}`);
+		await goto(resolve(`/files/${d.root_folder_id}`));
 	}
 
 	onMount(() => {
@@ -83,7 +84,7 @@
 					<span class="drive-picker__name">{d.name}</span>
 				</button>
 				<a
-					href={`/config/drive/${d.id}`}
+					href={resolve(`/config/drive/${d.id}`)}
 					class="drive-picker__settings"
 					title={t('drive.settings_aria', 'Drive settings')}
 					aria-label={t('drive.settings_aria', 'Drive settings')}