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/sidebar/calculator-tab.svelte

@@ -15,7 +15,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 -->
 <script lang="ts">
 	import { getContext } from "svelte";
-	import { page } from "$app/state";
+	import { appScreen } from "$lib/app-nav.svelte";
 
 	import { i18n, type TranslationKey } from "$lib/i18n/index.svelte";
 	import {
@@ -67,7 +67,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 	// Reset the design when the active game changes; a no-op otherwise, so
 	// the design persists across tab switches within a game.
 	$effect(() => {
-		cs.ensureGame(page.params.id ?? "");
+		cs.ensureGame(appScreen.gameId ?? "");
 	});
 
 	const core = $derived(coreHandle?.core ?? null);