feat: add migration wizard for flat vault

On vault load, detects files in non-protected subfolders via
vault_health_check. Shows an amber banner offering to flatten them
to the vault root. After migration, reloads the vault automatically.
The banner can be dismissed without migrating.

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

src/components/FlatVaultMigrationBanner.tsx

@@ -0,0 +1,37 @@
+interface FlatVaultMigrationBannerProps {
+  strayFileCount: number
+  isMigrating: boolean
+  onMigrate: () => void
+  onDismiss: () => void
+}
+
+/**
+ * Banner shown when the vault has files in non-protected subfolders.
+ * Offers to flatten them to the vault root.
+ */
+export function FlatVaultMigrationBanner({ strayFileCount, isMigrating, onMigrate, onDismiss }: FlatVaultMigrationBannerProps) {
+  return (
+    <div className="flex items-center gap-3 px-4 py-2 bg-amber-50 dark:bg-amber-950 border-b border-amber-200 dark:border-amber-800 text-sm" data-testid="migration-banner">
+      <span className="flex-1 text-amber-800 dark:text-amber-200">
+        {strayFileCount} note{strayFileCount !== 1 ? 's' : ''} found in subfolders.
+        Flatten to vault root for consistent scanning.
+      </span>
+      <button
+        className="px-3 py-1 text-xs font-medium rounded bg-amber-600 text-white hover:bg-amber-700 disabled:opacity-50"
+        onClick={onMigrate}
+        disabled={isMigrating}
+        data-testid="migration-flatten-btn"
+      >
+        {isMigrating ? 'Migrating...' : 'Flatten Now'}
+      </button>
+      <button
+        className="px-2 py-1 text-xs text-amber-600 dark:text-amber-400 hover:underline"
+        onClick={onDismiss}
+        disabled={isMigrating}
+        data-testid="migration-dismiss-btn"
+      >
+        Dismiss
+      </button>
+    </div>
+  )
+}