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/docs/README.md

@@ -31,6 +31,8 @@ lives in [`../PLAN.md`](../PLAN.md), the active web finalization in
   bridge, with the live function surface and parity rules.
 - [wasm-toolchain.md](wasm-toolchain.md) — building `ui/core` to
   `core.wasm` with TinyGo.
+- [pwa-strategy.md](pwa-strategy.md) — the installable/offline PWA: the
+  native service worker, manifest, icons, and the Playwright PWA gate.
 - [testing.md](testing.md) — the UI test layers (Vitest + Playwright).
 
 ## Auth & lobby