refactor: rearchitect navigation model with pure store and no sidebar pane

- Extract navigation state and logic into navigation-store.ts for
  testability and separation of concerns
- NavigationContext now only wraps the store as a Solid provider
- Remove SIDEBAR_PANE from the model; depth-tabs have a single
  focusable content pane (center/current column)
- Split LayerGraph and page imports into layer-graph.ts so the
  navigation utils stay free of JSX (unit-testable)
- Update Shell keybind dispatch: remove sidebar pane handling,
  simplify swipe to fixed-pane tabs only, clarify depth-tab h/l
- Add nav-model.test.ts covering depth stack, tab/pane focus, and
  selection semantics

src/utils/layer-graph.ts

@@ -0,0 +1,48 @@
+/**
+ * layer-graph — maps each TAB id to its page component + pane count.
+ *
+ * Split out of `navigation.ts` so that the nav-model primitives (TABS,
+ * TabsCount, DEPTH_TABS, rootFrameFor, TabPaneCount, PANE_RATIO) in
+ * `navigation.ts` stay free of any `.tsx` / JSX imports. This lets unit tests
+ * import the pure navigation store without pulling the OpenTUI JSX runtime
+ * (which is only provided by the build-time @opentui/solid bun-plugin).
+ *
+ * The page modules live alongside their pages and export `<count>PaneCount`
+ * constants describing how many focusable panes each fixed page owns.
+ */
+import {
+	DiscoverPage,
+	DiscoverPaneCount,
+} from "@/pages/Discover/DiscoverPage";
+import { FeedPage, FeedPaneCount } from "@/pages/Feed/FeedPage";
+import {
+	MyShowsPage,
+	MyShowsPaneCount,
+} from "@/pages/MyShows/MyShowsPage";
+import { PlayerPage, PlayerPaneCount } from "@/pages/Player/PlayerPage";
+import { SearchPage, SearchPaneCount } from "@/pages/Search/SearchPage";
+import {
+	SettingsPage,
+	SettingsPaneCount,
+} from "@/pages/Settings/SettingsPage";
+import { TABS } from "@/utils/navigation";
+
+/** Maps a TAB id to the page component that renders it. */
+export const LayerGraph = {
+	[TABS.FEED]: FeedPage,
+	[TABS.MYSHOWS]: MyShowsPage,
+	[TABS.DISCOVER]: DiscoverPage,
+	[TABS.SEARCH]: SearchPage,
+	[TABS.PLAYER]: PlayerPage,
+	[TABS.SETTINGS]: SettingsPage,
+};
+
+/** Per-tab focusable-pane counts (forwarded from each page's `*PaneCount`). */
+export const LayerDepths = {
+	[TABS.FEED]: FeedPaneCount,
+	[TABS.MYSHOWS]: MyShowsPaneCount,
+	[TABS.DISCOVER]: DiscoverPaneCount,
+	[TABS.SEARCH]: SearchPaneCount,
+	[TABS.PLAYER]: PlayerPaneCount,
+	[TABS.SETTINGS]: SettingsPaneCount,
+};