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.tsx

@@ -0,0 +1,48 @@
+import { Archive, ArrowUUpLeft } from '@phosphor-icons/react'
+
+interface ArchivedNoteBannerProps {
+  onUnarchive: () => void
+}
+
+export function ArchivedNoteBanner({ onUnarchive }: ArchivedNoteBannerProps) {
+  return (
+    <div
+      data-testid="archived-note-banner"
+      style={{
+        display: 'flex',
+        alignItems: 'center',
+        gap: 6,
+        padding: '4px 16px',
+        background: 'var(--muted)',
+        borderBottom: '1px solid var(--border)',
+        fontSize: 12,
+        color: 'var(--muted-foreground)',
+        flexShrink: 0,
+      }}
+    >
+      <Archive size={13} weight="bold" />
+      <span>Archived</span>
+      <button
+        data-testid="unarchive-btn"
+        onClick={onUnarchive}
+        style={{
+          display: 'inline-flex',
+          alignItems: 'center',
+          gap: 4,
+          marginLeft: 'auto',
+          padding: '2px 8px',
+          background: 'transparent',
+          border: '1px solid var(--border)',
+          borderRadius: 4,
+          fontSize: 11,
+          color: 'var(--muted-foreground)',
+          cursor: 'pointer',
+        }}
+        title="Unarchive (Cmd+E)"
+      >
+        <ArrowUUpLeft size={12} />
+        Unarchive
+      </button>
+    </div>
+  )
+}