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/i18n.md

@@ -85,7 +85,7 @@ helper is platform-agnostic by design.
 The boot locale resolves once at module load (no async init):
 an explicit stored choice wins, otherwise browser/system detection,
 otherwise `DEFAULT_LOCALE`. Callers that mutate the locale (the language
-pickers on `/login` and in the account menu) call `i18n.setLocale(next)`,
+pickers on the login screen and in the account menu) call `i18n.setLocale(next)`,
 which **persists** the choice to `localStorage` (key `galaxy-locale`) so
 it survives reloads. An unrecognised stored value is ignored and falls
 back to detection.