feat: add emoji icon picker for notes stored in frontmatter

Every note can now have an optional emoji icon (frontmatter `icon` field).
The icon is displayed in the editor header, note list, search results,
and Quick Open. Includes command palette commands "Set Note Icon" and
"Remove Note Icon", plus full test coverage (Vitest + Playwright).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/useCommandRegistry.test.ts

@@ -146,6 +146,51 @@ describe('useCommandRegistry', () => {
     rerender(makeConfig())
     expect(findCommand(result.current, 'resolve-conflicts')!.enabled).toBe(true)
   })
+
+  it('includes set-note-icon command in Note group', () => {
+    const config = makeConfig({ onSetNoteIcon: vi.fn() })
+    const { result } = renderHook(() => useCommandRegistry(config))
+    const cmd = findCommand(result.current, 'set-note-icon')
+    expect(cmd).toBeDefined()
+    expect(cmd!.group).toBe('Note')
+    expect(cmd!.label).toBe('Set Note Icon')
+  })
+
+  it('set-note-icon is enabled when active note and callback exist', () => {
+    const config = makeConfig({ onSetNoteIcon: vi.fn() })
+    const { result } = renderHook(() => useCommandRegistry(config))
+    const cmd = findCommand(result.current, 'set-note-icon')
+    expect(cmd!.enabled).toBe(true)
+  })
+
+  it('set-note-icon is disabled when no active note', () => {
+    const config = makeConfig({ activeTabPath: null, onSetNoteIcon: vi.fn() })
+    const { result } = renderHook(() => useCommandRegistry(config))
+    const cmd = findCommand(result.current, 'set-note-icon')
+    expect(cmd!.enabled).toBe(false)
+  })
+
+  it('remove-note-icon is enabled when active note has icon', () => {
+    const config = makeConfig({ onRemoveNoteIcon: vi.fn(), activeNoteHasIcon: true })
+    const { result } = renderHook(() => useCommandRegistry(config))
+    const cmd = findCommand(result.current, 'remove-note-icon')
+    expect(cmd!.enabled).toBe(true)
+  })
+
+  it('remove-note-icon is disabled when active note has no icon', () => {
+    const config = makeConfig({ onRemoveNoteIcon: vi.fn(), activeNoteHasIcon: false })
+    const { result } = renderHook(() => useCommandRegistry(config))
+    const cmd = findCommand(result.current, 'remove-note-icon')
+    expect(cmd!.enabled).toBe(false)
+  })
+
+  it('set-note-icon executes callback', () => {
+    const onSetNoteIcon = vi.fn()
+    const config = makeConfig({ onSetNoteIcon })
+    const { result } = renderHook(() => useCommandRegistry(config))
+    findCommand(result.current, 'set-note-icon')!.execute()
+    expect(onSetNoteIcon).toHaveBeenCalled()
+  })
 })
 
 describe('pluralizeType', () => {