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>

Dockerfile

@@ -9,7 +9,7 @@ RUN apk --no-cache upgrade && \
 # ─── Stage 1b: Build the SvelteKit frontend (Vite) ───────────────────────────
 # Produces the SPA in /static-dist. `npm ci` is cached unless the lockfile
 # changes; the Rust build no longer bundles assets (see build.rs).
-FROM node:24-alpine AS frontend
+FROM node:26.3.1-alpine3.24 AS frontend
 WORKDIR /frontend
 COPY frontend/package.json frontend/package-lock.json ./
 RUN npm ci