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/events.md

@@ -93,11 +93,11 @@ reconnect.
    });
    onDestroy(off);
    ```
-2. If the handler reads scoped data (per-game, per-route), register
-   from a layout that owns that scope and pass the gameId via a
-   closure. The handler should filter events whose payload does not
-   match its scope (see `routes/games/[id]/+layout.svelte` for the
-   `game.turn.ready` filter).
+2. If the handler reads scoped data (per-game), register from a
+   component that owns that scope and pass the gameId via a closure.
+   The handler should filter events whose payload does not match its
+   scope (see `lib/game/game-shell.svelte` for the `game.turn.ready`
+   filter).
 3. The payload encoding is owned by the producer side: the
    `game.turn.ready` event uses JSON `{game_id, turn}`. Document
    the schema next to the producer (e.g. `backend/README.md` §10).