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/composables/useOwnerCache.svelte.ts

@@ -31,6 +31,9 @@ export class OwnerCache {
 
 	/** Resolve every not-yet-cached id in parallel; nullish ids are skipped. */
 	async resolve(ids: Iterable<string | null | undefined>): Promise<void> {
+		// Transient scratch Set for dedup only — built, spread to an array, and
+		// discarded in this call; never read reactively, so a plain Set is correct.
+		// eslint-disable-next-line svelte/prefer-svelte-reactivity
 		const pending = [...new Set([...ids].filter((id): id is string => !!id))].filter(
 			(id) => !this.#names[id]
 		);

frontend/src/lib/composables/useSelection.svelte.ts

@@ -1,15 +1,18 @@
+import { SvelteSet } from 'svelte/reactivity';
+
 /**
  * Reactive multi-select over string ids. Backs the repeated
  * `let selected = $state(new Set()); function toggle(id) { … }` pattern used by
  * the photos grid, music picker and other list views with one source of truth.
  *
- * Mutations swap in a fresh Set so `$derived`/template reads re-run.
+ * Backed by a reactive {@link SvelteSet}, so in-place mutations (`add`/`delete`)
+ * drive `$derived`/template reads without copying the set.
  */
 export class Selection {
-	#ids = $state<Set<string>>(new Set());
+	#ids = new SvelteSet<string>();
 
 	/** The live selection set (read-only intent — mutate via the methods). */
-	get ids(): Set<string> {
+	get ids(): SvelteSet<string> {
 		return this.#ids;
 	}
 
@@ -31,31 +34,26 @@ export class Selection {
 	}
 
 	toggle(id: string): void {
-		const next = new Set(this.#ids);
-		if (next.has(id)) next.delete(id);
-		else next.add(id);
-		this.#ids = next;
+		if (this.#ids.has(id)) this.#ids.delete(id);
+		else this.#ids.add(id);
 	}
 
 	add(id: string): void {
-		if (this.#ids.has(id)) return;
-		this.#ids = new Set(this.#ids).add(id);
+		this.#ids.add(id);
 	}
 
 	delete(id: string): void {
-		if (!this.#ids.has(id)) return;
-		const next = new Set(this.#ids);
-		next.delete(id);
-		this.#ids = next;
+		this.#ids.delete(id);
 	}
 
 	/** Replace the whole selection. */
 	set(ids: Iterable<string>): void {
-		this.#ids = new Set(ids);
+		this.#ids.clear();
+		for (const id of ids) this.#ids.add(id);
 	}
 
 	clear(): void {
-		if (this.#ids.size) this.#ids = new Set();
+		this.#ids.clear();
 	}
 }