Initial commit: webmail client (M0/M1)

Three-pane webmail client.
- Backend: FastAPI + async SQLAlchemy + arq worker (Postgres/Redis), IMAP/SMTP
  sync, threading, search, drafts/outbox with undo, attachments, 2FA.
- Frontend: React 18 + Vite + TypeScript + TanStack Query + Tailwind; three-pane
  UI with rich-text composer, light/dark themes, resizable panels.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/Resizer.tsx

@@ -0,0 +1,32 @@
+// A vertical drag handle for adjusting the width of the panel to its left.
+export function Resizer({ onDelta }: { onDelta: (deltaX: number) => void }) {
+  const onMouseDown = (e: React.MouseEvent) => {
+    e.preventDefault();
+    let lastX = e.clientX;
+    const onMove = (ev: MouseEvent) => {
+      onDelta(ev.clientX - lastX);
+      lastX = ev.clientX;
+    };
+    const onUp = () => {
+      document.removeEventListener("mousemove", onMove);
+      document.removeEventListener("mouseup", onUp);
+      document.body.style.userSelect = "";
+      document.body.style.cursor = "";
+    };
+    document.addEventListener("mousemove", onMove);
+    document.addEventListener("mouseup", onUp);
+    document.body.style.userSelect = "none";
+    document.body.style.cursor = "col-resize";
+  };
+
+  return (
+    <div
+      onMouseDown={onMouseDown}
+      className="group relative z-20 -mx-0.5 w-1 shrink-0 cursor-col-resize"
+      role="separator"
+      aria-orientation="vertical"
+    >
+      <div className="absolute inset-y-0 left-1/2 w-px -translate-x-1/2 bg-transparent group-hover:bg-app-accent" />
+    </div>
+  );
+}