feat: barebones MVP — editor, live LSP, and preview

Backend (typst-leaf-backend)
- REST API for multi-project file management under PROJECTS_ROOT
  (list/create projects, get file tree, read/write files; zod-validated
  sandbox prevents path traversal).
- LSP thin proxy: spawns a single tinymist lsp process, pipes one active
  WebSocket straight to its stdio via vscode-ws-jsonrpc. New connections
  trigger a takeover (old session disposed, fresh tinymist spawned).
- Live preview pipeline: the frontend sends tinymist.startDefaultPreview
  over the LSP wire, reads the returned data-plane port, and relays it to
  the backend. The backend proxies /preview (WS, with retry and an Origin
  header for typst-preview) and /preview-app (HTTP, injecting a script that
  rewrites the webview's WS path to /preview and forces light mode).
- Graceful SIGTERM/SIGINT shutdown.
- Auto-seeds a hello project on first boot.
- Removed execa dependency (no longer needed).

Frontend (typst-leaf-frontend)
- Monaco editor with typst syntax highlighting (Monarch tokenizer) and
  live LSP diagnostics (markers), completion, hover, definitions, and
  formatting — all driven by a raw JSON-RPC WebSocket client (no
  monaco-languageclient or vscode-api dependency).
- Project picker and file tree sidebar.
- Live preview pane: iframes the typst-preview webview through the
  backend's /preview-app proxy, so the pipedepeline works remotely.
- Monaco bundled locally (no CDN) with a Vite worker setup.
- Vite dev proxy so relative /api, /lsp, /preview, and /preVIEW-app URLs
  reach the backend on port 4000.
- Light-mode UI (editor 'vs' theme, white chrome).
- Prevent esbuild from tree-shaking monaco.editor services
  (optimizeDeps.esbuildOptions.treeShaking: false).

Infrastructure
- Added vscode-ws-jsonrpc to backend dependencies.
- Removed monaco-languageclient and vscode-ws-jsonrpc from frontend
  dependencies (replaced by the raw JSON-RPC client).
- Added monaco-editor as a direct frontend dependency.
- Updated README (correct frontend port 1420, tinymist install command,
  environment variables, architecture).
- Added projects/ to .gitignore.
- Renamed Tauri productName and page title to typst-leaf.

typst-leaf-frontend/src/App.tsx

@@ -1,5 +1,48 @@
-function App() {
-  return <div className="h-screen w-screen bg-zinc-900 text-zinc-100" />;
+import { AppProvider, useApp } from "./app-state";
+import { EditorView } from "./components/Editor";
+import { FileTree } from "./components/FileTree";
+import { Preview } from "./components/Preview";
+import { ProjectPicker } from "./components/ProjectPicker";
+
+function Shell() {
+  const { state, lsp } = useApp();
+  return (
+    <div className="h-screen w-screen bg-white text-zinc-800 flex">
+      <aside className="w-60 border-r border-zinc-200 flex flex-col bg-zinc-50">
+        <div className="px-3 py-2 border-b border-zinc-200 text-sm font-semibold flex items-center gap-2">
+          <span>typst-leaf</span>
+          <span className="ml-auto text-xs font-normal text-zinc-400">
+            {lsp.status === "ready"
+              ? "●"
+              : lsp.status === "connecting"
+                ? "◌"
+                : lsp.status === "error"
+                  ? "✕"
+                  : "○"}
+          </span>
+        </div>
+        <ProjectPicker />
+        <div className="flex-1 overflow-auto">
+          <FileTree />
+        </div>
+        {state.error && (
+          <div className="p-2 text-xs text-red-500 border-t border-zinc-200">
+            {state.error}
+          </div>
+        )}
+      </aside>
+      <main className="flex-1 flex min-w-0">
+        <EditorView />
+        <Preview />
+      </main>
+    </div>
+  );
 }
 
-export default App;
+export default function App() {
+  return (
+    <AppProvider>
+      <Shell />
+    </AppProvider>
+  );
+}