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/static/manifest.webmanifest

@@ -1,42 +1,42 @@
 {
-  "name": "OxiCloud",
-  "short_name": "OxiCloud",
-  "description": "Fast, private cloud storage built with Rust.",
-  "start_url": "/",
-  "scope": "/",
-  "display": "standalone",
-  "background_color": "#0f172a",
-  "theme_color": "#0f172a",
-  "icons": [
-    {
-      "src": "/logo/logo-plain.svg",
-      "sizes": "any",
-      "type": "image/svg+xml",
-      "purpose": "any"
-    },
-    {
-      "src": "/logo/logo-maskable.svg",
-      "sizes": "any",
-      "type": "image/svg+xml",
-      "purpose": "maskable"
-    },
-    {
-      "src": "/logo/maskable-192.png",
-      "sizes": "192x192",
-      "type": "image/png",
-      "purpose": "maskable"
-    },
-    {
-      "src": "/logo/maskable-512.png",
-      "sizes": "512x512",
-      "type": "image/png",
-      "purpose": "maskable"
-    },
-    {
-      "src": "/logo/maskable-512.png",
-      "sizes": "512x512",
-      "type": "image/png",
-      "purpose": "any"
-    }
-  ]
+	"name": "OxiCloud",
+	"short_name": "OxiCloud",
+	"description": "Fast, private cloud storage built with Rust.",
+	"start_url": "/",
+	"scope": "/",
+	"display": "standalone",
+	"background_color": "#0f172a",
+	"theme_color": "#0f172a",
+	"icons": [
+		{
+			"src": "/logo/logo-plain.svg",
+			"sizes": "any",
+			"type": "image/svg+xml",
+			"purpose": "any"
+		},
+		{
+			"src": "/logo/logo-maskable.svg",
+			"sizes": "any",
+			"type": "image/svg+xml",
+			"purpose": "maskable"
+		},
+		{
+			"src": "/logo/maskable-192.png",
+			"sizes": "192x192",
+			"type": "image/png",
+			"purpose": "maskable"
+		},
+		{
+			"src": "/logo/maskable-512.png",
+			"sizes": "512x512",
+			"type": "image/png",
+			"purpose": "maskable"
+		},
+		{
+			"src": "/logo/maskable-512.png",
+			"sizes": "512x512",
+			"type": "image/png",
+			"purpose": "any"
+		}
+	]
 }