feat(ui): installable offline PWA — service worker, manifest, icons (F5)

Native SvelteKit service worker (src/service-worker.ts): a version-keyed
cache precaches the app shell + build artefacts (incl. core.wasm) +
static files; activate purges old caches; the gateway is never
intercepted; navigations fall back to the cached shell offline. Adds
static/manifest.webmanifest, a generated placeholder icon set
(scripts/gen-pwa-icons.mjs — dependency-free pure-Node PNG encoder), and
manifest / theme-color / apple-touch tags in app.html.

Gated by Playwright against a production preview (playwright.pwa.config.ts
+ tests/pwa/pwa.spec.ts via `pnpm test:pwa`, wired into ui-test):
manifest + installable icons, SW registration + a single version-keyed
cache, and offline shell load. Lighthouse is not used — its PWA category
was removed in v12.

Docs: ui/docs/pwa-strategy.md (+ index); F5 marked done.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/ui-test.yaml

@@ -98,6 +98,12 @@ jobs:
         working-directory: ui/frontend
         run: pnpm exec playwright test
 
+      - name: Run PWA tests
+        # Builds + previews the production bundle (the service worker only
+        # precaches a real build) and checks manifest / SW / offline.
+        working-directory: ui/frontend
+        run: pnpm test:pwa
+
       - name: Upload Playwright report on failure
         if: failure()
         uses: actions/upload-artifact@v4