feat: show archived note indicator banner in editor (#183)

Adds a subtle ArchivedNoteBanner component below the breadcrumb bar
when a note is archived. Banner includes:
- Muted gray background with archive icon + 'Archived' label
- 'Unarchive' button (ArrowUUpLeft icon) wired to same handler as Cmd+E
- Keyboard hint shown in button title

Editor remains fully editable (banner is purely informational).
Indicator appears/disappears reactively via entry.archived from store.

Co-authored-by: Test <test@test.com>

src/components/ArchivedNoteBanner.test.tsx

@@ -0,0 +1,26 @@
+import { render, screen, fireEvent } from '@testing-library/react'
+import { describe, it, expect, vi } from 'vitest'
+import { ArchivedNoteBanner } from './ArchivedNoteBanner'
+
+describe('ArchivedNoteBanner', () => {
+  it('renders archive icon and label', () => {
+    render(<ArchivedNoteBanner onUnarchive={vi.fn()} />)
+    expect(screen.getByTestId('archived-note-banner')).toBeTruthy()
+    expect(screen.getByText('Archived')).toBeTruthy()
+  })
+
+  it('renders unarchive button with keyboard hint', () => {
+    render(<ArchivedNoteBanner onUnarchive={vi.fn()} />)
+    const btn = screen.getByTestId('unarchive-btn')
+    expect(btn).toBeTruthy()
+    expect(btn.textContent).toContain('Unarchive')
+    expect(btn.title).toBe('Unarchive (Cmd+E)')
+  })
+
+  it('calls onUnarchive when button is clicked', () => {
+    const onUnarchive = vi.fn()
+    render(<ArchivedNoteBanner onUnarchive={onUnarchive} />)
+    fireEvent.click(screen.getByTestId('unarchive-btn'))
+    expect(onUnarchive).toHaveBeenCalledOnce()
+  })
+})