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/components/Preview.tsx

@@ -0,0 +1,39 @@
+import { useApp } from "../app-state";
+
+export function Preview() {
+  const { state, lsp } = useApp();
+
+  let status = "idle";
+  if (!state.currentProject) status = "no project";
+  else if (lsp.status !== "ready") status = `LSP ${lsp.status}`;
+  else if (!lsp.previewReady) status = "starting preview…";
+  else status = "live";
+
+  return (
+    <div className="w-1/2 border-l border-zinc-200 flex flex-col bg-white">
+      <div className="px-3 py-1.5 border-b border-zinc-200 text-xs text-zinc-500 font-mono flex items-center gap-2">
+        <span>Preview</span>
+        <span className="ml-auto text-zinc-400">{status}</span>
+      </div>
+      <div className="flex-1 bg-zinc-50 relative">
+        {state.currentProject && lsp.status === "ready" && lsp.previewReady ? (
+          <iframe
+            key={state.currentProject}
+            src="/preview-app"
+            title="Typst preview"
+            className="absolute inset-0 w-full h-full bg-white"
+          />
+        ) : (
+          <div className="absolute inset-0 flex flex-col items-center justify-center text-zinc-400 text-sm p-6 gap-2">
+            <span>{status}</span>
+            {lsp.status === "error" && lsp.error && (
+              <pre className="mt-2 max-w-full overflow-auto text-xs text-red-500 bg-red-50 p-3 rounded whitespace-pre-wrap break-all">
+                {lsp.error}
+              </pre>
+            )}
+          </div>
+        )}
+      </div>
+    </div>
+  );
+}