docs(ui): sync docs to the app-shell; fix stale nav comments

Rewrite ui/docs (navigation, order-composer, auth-flow, pwa-strategy,
game-state + secondary topic docs) and ui/README for the single-URL
app-shell (in-memory screens/views, Back→lobby via shallow routing,
sessionStorage restore + validation, return-to-lobby). ui/PLAN.md gets a
Phase-10 supersede note (implemented; standalone-compatible). Fix stale
code comments (session-store auth gate, report-sections spec contract).

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

ui/docs/pwa-strategy.md

@@ -4,12 +4,21 @@ 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.
 
+The single-URL app-shell (see [`navigation.md`](navigation.md)) makes
+the offline story simpler: the whole game UI lives at one route
+(`${base}/`, i.e. `/game/` under the single-origin deployment), so
+there is exactly one navigation target to precache and fall back to —
+no per-screen routes to enumerate. The service-worker scope and the
+manifest are unchanged by that refactor; both were already base-aware
+(the SW keys everything off `$service-worker`'s `base`, and the
+manifest uses relative `./` `start_url` / `scope`).
+
 ## 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
+  It precaches the app shell (`${base}/`), the build artefacts (JS/CSS
+  + `core.wasm`), and the static files under a **version-keyed** cache
   (`galaxy-cache-<version>`, `version` from `$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;
@@ -17,7 +26,8 @@ build dependency and the cache logic stays explicit.
   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
+  — name, `standalone` display, relative `./` `start_url`/`scope` (so
+  it resolves under whatever `base` the build is deployed at), 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