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/static/manifest.webmanifest

@@ -0,0 +1,32 @@
+{
+	"name": "Galaxy",
+	"short_name": "Galaxy",
+	"description": "Galaxy — a turn-based space strategy game.",
+	"id": "/",
+	"start_url": "/",
+	"scope": "/",
+	"display": "standalone",
+	"orientation": "any",
+	"background_color": "#0a0e1a",
+	"theme_color": "#0a0e1a",
+	"icons": [
+		{
+			"src": "/icons/icon-192.png",
+			"sizes": "192x192",
+			"type": "image/png",
+			"purpose": "any"
+		},
+		{
+			"src": "/icons/icon-512.png",
+			"sizes": "512x512",
+			"type": "image/png",
+			"purpose": "any"
+		},
+		{
+			"src": "/icons/icon-maskable-512.png",
+			"sizes": "512x512",
+			"type": "image/png",
+			"purpose": "maskable"
+		}
+	]
+}