test: add Playwright smoke tests for flat vault migration

Covers: TitleField visibility, filename indicator on focus, no
migration banner when vault is flat, CSS rule hiding H1 in editor.

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

tests/smoke/migrate-to-flat-vault.spec.ts

@@ -0,0 +1,62 @@
+import { test, expect } from '@playwright/test'
+
+test.describe('Flat vault migration', () => {
+  test.beforeEach(async ({ page }) => {
+    await page.goto('/')
+    await page.waitForLoadState('networkidle')
+  })
+
+  test('title field appears above editor when note is open', async ({ page }) => {
+    // Click a note in the sidebar to open it
+    const noteItem = page.locator('[data-testid="note-list-item"]').first()
+    if (await noteItem.isVisible()) {
+      await noteItem.click()
+      await page.waitForTimeout(300)
+      // The TitleField should be visible
+      const titleField = page.locator('[data-testid="title-field"]')
+      await expect(titleField).toBeVisible()
+      // The input should have the note's title
+      const input = page.locator('[data-testid="title-field-input"]')
+      const value = await input.inputValue()
+      expect(value.length).toBeGreaterThan(0)
+    }
+  })
+
+  test('title field is editable and shows filename on change', async ({ page }) => {
+    // Open a note
+    const noteItem = page.locator('[data-testid="note-list-item"]').first()
+    if (await noteItem.isVisible()) {
+      await noteItem.click()
+      await page.waitForTimeout(300)
+      const input = page.locator('[data-testid="title-field-input"]')
+      await input.focus()
+      // Should show filename indicator when focused
+      const filenameIndicator = page.locator('[data-testid="title-field-filename"]')
+      await expect(filenameIndicator).toBeVisible()
+    }
+  })
+
+  test('no migration banner when vault is already flat (mock)', async ({ page }) => {
+    // In browser mode (mock), vault should be flat and no migration banner
+    const banner = page.locator('[data-testid="migration-banner"]')
+    await page.waitForTimeout(1000)
+    await expect(banner).not.toBeVisible()
+  })
+
+  test('H1 heading is hidden in editor (CSS rule active)', async ({ page }) => {
+    // Check that the CSS rule for hiding H1 is present in the document
+    const styles = await page.evaluate(() => {
+      for (const sheet of document.styleSheets) {
+        try {
+          for (const rule of sheet.cssRules) {
+            if (rule.cssText?.includes('data-content-type="heading"') && rule.cssText?.includes('display: none')) {
+              return true
+            }
+          }
+        } catch { /* cross-origin */ }
+      }
+      return false
+    })
+    expect(styles).toBe(true)
+  })
+})