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>

ui/frontend/package.json

@@ -9,7 +9,8 @@
     "preview": "vite preview",
     "check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
     "test": "vitest run",
-    "test:e2e": "playwright test"
+    "test:e2e": "playwright test",
+    "test:pwa": "playwright test --config playwright.pwa.config.ts"
   },
   "dependencies": {
     "flatbuffers": "^25.9.23",