feat(ui): numbered context-panel surface switching with configurable prefix

- Add switch_context_surface shortcut (default Cmd/Ctrl + 1..9, 0 for the 10th
  surface) that opens/closes/switches context panel rail surfaces by their
  visible order, configurable and persisted in Settings -> Shortcuts.
- Show order-number badges on rail icons while the modifier is held >500ms;
  dismiss on release, blur, or a number press until the next press-and-hold.
- Remove the legacy mod+2/3/4 (diff/terminal/git) and switch_tab_1..9 bindings
  so numbered surface switching goes only through the new mechanism.
- Replace the help-dialog 'Switch Project' row with the surface-switch row and
  update the shortcuts footer/header icons to the command icon.

packages/ui/src/lib/surfaces/DOCUMENTATION.md

@@ -20,6 +20,13 @@ edge (`components/layout/ContextPanelRail.tsx`) and rendered by
 - Rail order is user-reorderable and persisted globally in
   `useUIStore.contextRailOrder`; `sortContextSurfaces` applies it on top of the
   registry's default order and appends any missing surfaces.
+- `getVisibleContextRailSurfaces` is the single visibility filter shared by the
+  rail and the global surface-switch shortcut (`switch_context_surface` in
+  `lib/shortcuts.ts`): it drops the plan surface unless plan mode is enabled,
+  drops the walkthrough on VS Code and below `WALKTHROUGH_MIN_WIDTH`, and hides
+  `has-content` surfaces until a tab of their mode exists. Both consumers use
+  it so the digit shown on a rail badge always maps to the same surface the
+  shortcut opens.
 
 ## Adding a surface
 

packages/ui/src/lib/surfaces/registry.test.ts

@@ -0,0 +1,53 @@
+import { describe, expect, test } from 'bun:test';
+
+import {
+  CONTEXT_SURFACES,
+  getVisibleContextRailSurfaces,
+  WALKTHROUGH_MIN_WIDTH,
+} from './registry';
+
+const baseOptions = {
+  railOrder: [],
+  planModeEnabled: true,
+  isVSCode: false,
+  screenWidth: 1200,
+  tabs: [],
+} as const;
+
+describe('getVisibleContextRailSurfaces', () => {
+  test('hides the plan surface while plan mode is disabled', () => {
+    const surfaces = getVisibleContextRailSurfaces({ ...baseOptions, planModeEnabled: false });
+    expect(surfaces.some((surface) => surface.id === 'plan')).toBe(false);
+    expect(surfaces.some((surface) => surface.id === 'context')).toBe(true);
+  });
+
+  test('shows the plan surface while plan mode is enabled', () => {
+    const surfaces = getVisibleContextRailSurfaces({ ...baseOptions, planModeEnabled: true });
+    expect(surfaces.some((surface) => surface.id === 'plan')).toBe(true);
+  });
+
+  test('hides the walkthrough on VS Code and below the min width', () => {
+    expect(getVisibleContextRailSurfaces({ ...baseOptions, isVSCode: true }).some((s) => s.id === 'walkthrough')).toBe(false);
+    expect(
+      getVisibleContextRailSurfaces({ ...baseOptions, screenWidth: WALKTHROUGH_MIN_WIDTH - 1 }).some((s) => s.id === 'walkthrough'),
+    ).toBe(false);
+    expect(
+      getVisibleContextRailSurfaces({ ...baseOptions, screenWidth: WALKTHROUGH_MIN_WIDTH }).some((s) => s.id === 'walkthrough'),
+    ).toBe(true);
+  });
+
+  test('hides content-driven surfaces until a matching tab exists', () => {
+    const preview = CONTEXT_SURFACES.find((surface) => surface.id === 'preview');
+    if (!preview) {
+      throw new Error('preview surface missing from registry');
+    }
+    expect(preview.availability).toBe('has-content');
+    expect(getVisibleContextRailSurfaces(baseOptions).some((s) => s.id === 'preview')).toBe(false);
+    expect(getVisibleContextRailSurfaces({ ...baseOptions, tabs: [{ mode: preview.mode }] }).some((s) => s.id === 'preview')).toBe(true);
+  });
+
+  test('respects the persisted user rail order', () => {
+    const surfaces = getVisibleContextRailSurfaces({ ...baseOptions, railOrder: ['git', 'context'] });
+    expect(surfaces.slice(0, 2).map((surface) => surface.id)).toEqual(['git', 'context']);
+  });
+});

packages/ui/src/lib/surfaces/registry.ts

@@ -152,6 +152,10 @@ export const CONTEXT_SURFACES: readonly ContextSurfaceDescriptor[] = [
 const SURFACE_BY_ID = new Map(CONTEXT_SURFACES.map((surface) => [surface.id, surface]));
 const FRACTION_BY_MODE = new Map(CONTEXT_SURFACES.map((surface) => [surface.mode, surface.defaultWidthFraction]));
 
+// Tablet width and up: below this the walkthrough cannot show a stop and its
+// code side by side, which is the whole point of the surface.
+export const WALKTHROUGH_MIN_WIDTH = 768;
+
 export const getContextSurfaceWidthFraction = (mode: ContextPanelMode): number => {
   return FRACTION_BY_MODE.get(mode) ?? 1 / 2;
 };
@@ -187,3 +191,36 @@ export const sortContextSurfaces = (railOrder: readonly string[]): ContextSurfac
 
   return ordered;
 };
+
+type VisibleRailSurfacesOptions = {
+  railOrder: readonly string[];
+  planModeEnabled: boolean;
+  isVSCode: boolean;
+  screenWidth: number;
+  tabs: readonly { mode: ContextPanelMode }[];
+};
+
+/**
+ * The context panel rail's visible, user-ordered surfaces. Shared by the rail
+ * (for rendering and number badges) and the global surface-switch shortcut so
+ * both agree on which surface each digit maps to.
+ *
+ * Content-driven surfaces are hidden (not disabled) until content exists; an
+ * existing tab keeps them visible even if the content source went away.
+ */
+export const getVisibleContextRailSurfaces = (options: VisibleRailSurfacesOptions): ContextSurfaceDescriptor[] => {
+  return sortContextSurfaces(options.railOrder).filter((surface) => {
+    if (surface.id === 'plan' && !options.planModeEnabled) {
+      return false;
+    }
+    // The walkthrough needs room for a stop list beside real code, and its
+    // diffs come from OpenChamber's Git routes, which VS Code does not serve.
+    if (surface.id === 'walkthrough' && (options.isVSCode || options.screenWidth < WALKTHROUGH_MIN_WIDTH)) {
+      return false;
+    }
+    if (surface.availability === 'has-content') {
+      return options.tabs.some((tab) => tab.mode === surface.mode);
+    }
+    return true;
+  });
+};