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

ui/docs/pwa-strategy.md

@@ -0,0 +1,46 @@
+# PWA strategy
+
+The web client is an installable, offline-tolerant PWA. It uses
+SvelteKit's native service worker (no Workbox) so there is no extra
+build dependency and the cache logic stays explicit.
+
+## Pieces
+
+- [`src/service-worker.ts`](../frontend/src/service-worker.ts) — the
+  worker. SvelteKit registers it automatically in the production build.
+  It precaches the app shell (`/`), the build artefacts (JS/CSS +
+  `core.wasm`), and the static files under a **version-keyed** cache
+  (`galaxy-cache-<version>`, `version` from `$service-worker`). On
+  `activate` it deletes every other cache, so a new deploy never serves
+  stale code. Strategy: cache-first for the version-keyed build/files;
+  network-first with cache fallback for everything else; the cached
+  shell answers navigations when fully offline. The gateway (cross-
+  origin) is never intercepted — it is always live network.
+- [`static/manifest.webmanifest`](../frontend/static/manifest.webmanifest)
+  — name, `standalone` display, `start_url`/`scope` `/`, dark
+  `theme_color`/`background_color`, and the icon set.
+- [`static/icons/`](../frontend/static/icons/) — `192`/`512` (`any`),
+  a `512` `maskable`, and a `180` apple-touch icon. They are placeholder
+  artwork generated from `static/favicon.svg` by
+  [`scripts/gen-pwa-icons.mjs`](../frontend/scripts/gen-pwa-icons.mjs)
+  (a dependency-free pure-Node PNG encoder); swap in real artwork at the
+  same paths and the manifest is unchanged.
+- [`src/app.html`](../frontend/src/app.html) — the manifest link, the
+  apple-touch-icon link, and light/dark `theme-color` metas matching the
+  design tokens.
+
+## Testing
+
+PWA behaviour is gated by Playwright against a **production preview**
+build ([`playwright.pwa.config.ts`](../frontend/playwright.pwa.config.ts)
++ [`tests/pwa/pwa.spec.ts`](../frontend/tests/pwa/pwa.spec.ts), run by
+`pnpm test:pwa` in CI): the manifest is linked with installable icons,
+the service worker registers and controls the page under exactly one
+version-keyed cache, and the app shell loads offline from that cache.
+The spec needs a real build because `$service-worker`'s `build` list is
+empty under `vite dev`.
+
+Lighthouse is intentionally **not** used: its PWA category was removed
+in Lighthouse 12 (the current line is 13.x), so "Lighthouse PWA ≥ 90" is
+no longer a meaningful gate. The Playwright checks above verify the same
+install/offline behaviour directly.