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/header/header.svelte

@@ -27,11 +27,10 @@ absent until Phase 24 wires push-event state.
 	import TurnNavigator from "./turn-navigator.svelte";
 
 	type Props = {
-		gameId: string;
 		sidebarOpen: boolean;
 		onToggleSidebar: () => void;
 	};
-	let { gameId, sidebarOpen, onToggleSidebar }: Props = $props();
+	let { sidebarOpen, onToggleSidebar }: Props = $props();
 
 	const gameState = getContext<GameStateStore | undefined>(
 		GAME_STATE_CONTEXT_KEY,
@@ -69,7 +68,7 @@ absent until Phase 24 wires push-event state.
 		>
 			⤧
 		</button>
-		<ViewMenu {gameId} />
+		<ViewMenu />
 		<AccountMenu />
 	</div>
 </header>

ui/frontend/src/lib/header/view-menu.svelte

@@ -7,21 +7,18 @@ itself is identical. The same component is reused for the mobile
 
 Lists the seven IA destinations: map, tables (sub-list of six
 entities), report, battle, mail, ship-class designer, science
-designer. Closes on Escape, on outside click, and after a
-navigation. Phase 26 introduces the history-mode entry; Phase 35
-polishes microcopy.
+designer. Each entry mutates `activeView` (the single-URL app-shell
+has no per-view routes) and closes the menu. Closes on Escape, on
+outside click, and after a selection. Phase 26 introduces the
+history-mode entry; Phase 35 polishes microcopy.
 -->
 <script lang="ts">
-	import { withBase } from "$lib/paths";
 	import { onMount } from "svelte";
-	import { goto } from "$app/navigation";
+	import { activeView, type GameView } from "$lib/app-nav.svelte";
 	import { i18n, type TranslationKey } from "$lib/i18n/index.svelte";
 	import { mailStore } from "$lib/mail-store.svelte";
 	import { restoreFocus } from "$lib/a11y/restore-focus";
 
-	type Props = { gameId: string };
-	let { gameId }: Props = $props();
-
 	const mailUnread = $derived(mailStore.unreadCount);
 
 	let open = $state(false);
@@ -40,9 +37,12 @@ polishes microcopy.
 		open = !open;
 	}
 
-	function go(path: string): void {
+	function select(
+		view: GameView,
+		params: { tableEntity?: string } = {},
+	): void {
 		open = false;
-		void goto(withBase(path));
+		activeView.select(view, params);
 	}
 
 	function onKeyDown(event: KeyboardEvent): void {
@@ -93,7 +93,7 @@ polishes microcopy.
 				type="button"
 				role="menuitem"
 				data-testid="view-menu-item-map"
-				onclick={() => go(`/games/${gameId}/map`)}
+				onclick={() => select("map")}
 			>
 				{i18n.t("game.view.map")}
 			</button>
@@ -105,7 +105,7 @@ polishes microcopy.
 							type="button"
 							role="menuitem"
 							data-testid="view-menu-item-table-{entry.slug}"
-							onclick={() => go(`/games/${gameId}/table/${entry.slug}`)}
+							onclick={() => select("table", { tableEntity: entry.slug })}
 						>
 							{i18n.t(entry.key)}
 						</button>
@@ -116,7 +116,7 @@ polishes microcopy.
 				type="button"
 				role="menuitem"
 				data-testid="view-menu-item-report"
-				onclick={() => go(`/games/${gameId}/report`)}
+				onclick={() => select("report")}
 			>
 				{i18n.t("game.view.report")}
 			</button>
@@ -124,7 +124,7 @@ polishes microcopy.
 				type="button"
 				role="menuitem"
 				data-testid="view-menu-item-battle"
-				onclick={() => go(`/games/${gameId}/battle`)}
+				onclick={() => select("battle")}
 			>
 				{i18n.t("game.view.battle")}
 			</button>
@@ -133,7 +133,7 @@ polishes microcopy.
 				role="menuitem"
 				data-testid="view-menu-item-mail"
 				class="with-badge"
-				onclick={() => go(`/games/${gameId}/mail`)}
+				onclick={() => select("mail")}
 			>
 				<span>{i18n.t("game.view.mail")}</span>
 				{#if mailUnread > 0}
@@ -146,7 +146,7 @@ polishes microcopy.
 				type="button"
 				role="menuitem"
 				data-testid="view-menu-item-designer-science"
-				onclick={() => go(`/games/${gameId}/designer/science`)}
+				onclick={() => select("designer-science")}
 			>
 				{i18n.t("game.view.designer.science")}
 			</button>