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/components/Layout.jsx

@@ -1,4 +1,4 @@
-import { Link, Outlet, useLocation, useNavigate } from 'react-router-dom';
+import { Link, Outlet, useLocation, useNavigate, useSearchParams } from 'react-router-dom';
 import { useEffect, useState, useCallback } from 'react';
 import { AuthApi, SyncApi, AnalyticsApi } from '../api/client.js';
 import Logo from './Logo.jsx';
@@ -102,6 +102,8 @@ function FolderLink({ item, active, count, collapsed, extra }) {
 
 export default function Layout() {
   const [user, setUser] = useState(null);
+  const [searchParams] = useSearchParams();
+  const [searchQuery, setSearchQuery] = useState(() => searchParams.get('q') ?? '');
   const [sidebarOpen, setSidebarOpen] = useState(() => {
     try { return localStorage.getItem(LS_OPEN) !== 'false'; } catch { return true; }
   });
@@ -158,6 +160,12 @@ export default function Layout() {
     navigate('/');
   };
 
+  const submitSearch = (e) => {
+    e.preventDefault();
+    const q = searchQuery.trim();
+    if (q) navigate(`/app/search?q=${encodeURIComponent(q)}`);
+  };
+
   const startSync = async () => {
     try { await SyncApi.incremental(); } catch { /* ignore */ }
     if (loc.pathname !== '/app') navigate('/app');
@@ -199,6 +207,17 @@ export default function Layout() {
             <Link key={n.to} to={n.to} className={isNavActive(n) ? 'active' : ''}>{n.label}</Link>
           ))}
         </nav>
+        <form className="search-form" onSubmit={submitSearch}>
+          <input
+            className="search-input"
+            type="search"
+            placeholder="Search… (from:, is:unread, has:attachment)"
+            value={searchQuery}
+            onChange={(e) => setSearchQuery(e.target.value)}
+            aria-label="Search emails"
+          />
+          <button type="submit" className="search-btn" aria-label="Search">🔍</button>
+        </form>
         <div className="spacer" />
         <button onClick={startSync}>Sync now</button>
         <span className="user">{user?.email}</span>