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/diplomail-ui.md

@@ -1,9 +1,10 @@
 # In-game diplomatic mail UI
 
 The in-game mail view consumes the `diplomail` subsystem in the
-backend. The route lives at `/games/:id/mail` and replaces the
-active view when the user opens the "diplomatic mail" entry in the
-header menu.
+backend. It is the `mail` active view (`activeView.view === "mail"`)
+and replaces the active view when the user opens the "diplomatic mail"
+entry in the header menu (`activeView.select("mail")`). The app-shell
+has no per-view URL — see [`navigation.md`](navigation.md).
 
 ## Wire surface
 
@@ -70,11 +71,12 @@ render the original directly with no toggle.
 
 `diplomail.message.received` push frames are dispatched from
 `api/events.svelte.ts` via the singleton SubscribeEvents stream. The
-in-game layout (`routes/games/[id]/+layout.svelte`) parses the
+in-game shell (`lib/game/game-shell.svelte`) parses the
 verified payload, calls `mailStore.applyPushEvent(gameId)` (which
 re-fetches the inbox — the payload only carries a preview), and
-raises a toast through `lib/toast.svelte.ts` with a "view"
-deep-link to `/games/:id/mail`.
+raises a toast through `lib/toast.svelte.ts` whose "view" action
+switches to the mail view in memory (`activeView.select("mail")`) —
+no URL navigation.
 
 The header view-menu's mail entry shows `mailStore.unreadCount` as
 an inline pill — the only chrome the badge needs.