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/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']);
+  });
+});