feat: search bar in topbar with Gmail-style query syntax

- Search form in topbar routes to /app/search?q=...
- SearchResults page with infinite scroll, same row style as FolderView
- Supports Gmail-like operators: from:, is:unread, has:attachment, after:, before:
- SearchApi.query() calls GET /search which runs through the existing GmailQueryParser
- Input sanitised via encodeURIComponent on submit; external links use noopener/noreferrer

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/client.js

@@ -65,6 +65,8 @@ export const LayoutApi = {
 export const SearchApi = {
   folder: (slug, page = 1, pageSize = 50) =>
     api.post('/search', folderToRequest(slug, page, pageSize)).then((r) => r.data),
+  query: (q, page = 1, pageSize = 50) =>
+    api.get('/search', { params: { q, page, pageSize } }).then((r) => r.data),
 };
 
 function folderToRequest(slug, page, pageSize) {