feat(ui): app-shell core — single-route dispatcher, route collapse, nav→state

Collapse the game UI to one route (`/`): a screen dispatcher renders
login/lobby/lobby-create/game from `appScreen`/`activeView` state instead of
URL routes. Move screen components to lib/screens & lib/game; the game shell
reads the game id from `appScreen.gameId` and re-inits per-game stores via an
$effect; in-game views render from `activeView`. Flip ~23 goto/href nav sites
to store mutations; drop the `?sidebar=` URL coupling. Auth gate is now
state-based. WIP: browser-history (Back→lobby), restore-validation, the
return-to-lobby button, push deep-links, and the test migration are follow-ups
on this branch.

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

ui/frontend/src/lib/active-view/mail.svelte

@@ -6,7 +6,7 @@ pane, system-item pane, compose form) live under
 `./mail/*.svelte`.
 -->
 <script lang="ts">
-	import { page } from "$app/state";
+	import { appScreen } from "$lib/app-nav.svelte";
 
 	import { i18n } from "$lib/i18n/index.svelte";
 	import { mailStore, type MailListEntry } from "$lib/mail-store.svelte";
@@ -19,7 +19,7 @@ pane, system-item pane, compose form) live under
 	let selectedKey = $state<string | null>(null);
 	let composeOpen = $state(false);
 
-	const gameId = $derived(page.params.id ?? "");
+	const gameId = $derived(appScreen.gameId ?? "");
 
 	const entries = $derived(mailStore.entries);