feat(ext/vault): vault-status indicator renderer (Plan C Phase 6)

Renders sidebar-footer indicator with ahead/behind/pending state. Pure
DOM; reuses shared/glyphs (four new status glyphs) and shared/relative-time.
Status fetch happens in the wiring layer (Task 6.3).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/src/vault/__tests__/status-indicator.test.ts

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+import { describe, expect, it } from 'vitest';
+import { renderStatusIndicator } from '../vault-status';
+
+describe('vault status indicator', () => {
+  it('renders "in sync" when ahead/behind/pending all zero', () => {
+    const el = document.createElement('div');
+    renderStatusIndicator(el, { ahead: 0, behind: 0, lastSyncAt: 1700000000, pendingItems: 0 });
+    expect(el.textContent).toMatch(/in sync/i);
+  });
+
+  it('renders "N ahead" when ahead is non-zero', () => {
+    const el = document.createElement('div');
+    renderStatusIndicator(el, { ahead: 3, behind: 0, lastSyncAt: 1700000000, pendingItems: 0 });
+    expect(el.textContent).toMatch(/3 ahead/i);
+  });
+
+  it('renders "N behind" when behind is non-zero', () => {
+    const el = document.createElement('div');
+    renderStatusIndicator(el, { ahead: 0, behind: 2, lastSyncAt: 1700000000, pendingItems: 0 });
+    expect(el.textContent).toMatch(/2 behind/i);
+  });
+
+  it('renders "N pending" when pendingItems is non-zero', () => {
+    const el = document.createElement('div');
+    renderStatusIndicator(el, { ahead: 0, behind: 0, lastSyncAt: 1700000000, pendingItems: 5 });
+    expect(el.textContent).toMatch(/5 pending/i);
+  });
+
+  it('renders "never synced" when lastSyncAt is null', () => {
+    const el = document.createElement('div');
+    renderStatusIndicator(el, { ahead: 0, behind: 0, lastSyncAt: null, pendingItems: 0 });
+    expect(el.textContent).toMatch(/never synced/i);
+  });
+});