feat(ext/vault): fullscreen form header with dirty-state subtitle

Title left ('new login' / 'edit login'), subtitle below cycles between
'no changes' and 'unsaved · esc to cancel' on input events. Right side
shows the platform-aware save hint ('⌘+S to save' / 'Ctrl+S to save').
The actual ⌘+S keymap arrives in Phase 3 — this is a visual hint only.

extension/src/vault/__tests__/form-wrapper.test.ts

@@ -0,0 +1,45 @@
+import { describe, it, expect } from 'vitest';
+import * as fs from 'fs';
+import * as path from 'path';
+
+describe('fullscreen form dirty subtitle', () => {
+  const vaultSrc = fs.readFileSync(
+    path.resolve(__dirname, '../vault.ts'),
+    'utf-8',
+  );
+
+  it('contains renderFormWrapped function', () => {
+    expect(vaultSrc).toContain('function renderFormWrapped');
+  });
+
+  it('starts pristine: renders "no changes" subtitle', () => {
+    expect(vaultSrc).toContain("'no changes'");
+  });
+
+  it('switches to dirty on first input event', () => {
+    expect(vaultSrc).toContain("'unsaved · esc to cancel'");
+  });
+
+  it('listens on input and change events on the scroll element', () => {
+    expect(vaultSrc).toContain("scrollEl.addEventListener('input', markDirty, true)");
+    expect(vaultSrc).toContain("scrollEl.addEventListener('change', markDirty, true)");
+  });
+
+  it('marks clean on save', () => {
+    expect(vaultSrc).toContain('markClean()');
+  });
+
+  it('contains platform-aware SAVE_HINT', () => {
+    expect(vaultSrc).toContain('SAVE_HINT');
+    expect(vaultSrc).toContain('⌘+S to save');
+    expect(vaultSrc).toContain('Ctrl+S to save');
+  });
+
+  it('renders fullscreen-form-header element', () => {
+    expect(vaultSrc).toContain('fullscreen-form-header');
+  });
+
+  it('renders form-dirty-sub element', () => {
+    expect(vaultSrc).toContain('form-dirty-sub');
+  });
+});

extension/src/vault/vault.css

@@ -1606,6 +1606,29 @@ textarea {
   margin-bottom: 12px;
 }
 
+/* Phase 2B: fullscreen form header */
+.fullscreen-form-header {
+  padding: 14px 24px;
+  border-bottom: 1px solid var(--border-mid);
+  display: flex;
+  justify-content: space-between;
+  align-items: flex-start;
+}
+.fullscreen-form-header .title {
+  font-size: 16px;
+  font-weight: 500;
+  color: var(--text);
+}
+.fullscreen-form-header .sub {
+  font-size: 11px;
+  color: var(--text-muted);
+  margin-top: 2px;
+}
+.fullscreen-form-header .hint {
+  font-size: 11px;
+  color: var(--text-dim);
+}
+
 /* Phase 2B: sticky save bar + scrollable form pane */
 .form-pane {
   display: flex;