v2.0.0: BooCoder frontend — chat pane + diff pane + session picker

Phase 3 of v2.0. React + Vite SPA at apps/coder/web/ served by
the coder Fastify server via @fastify/static with SPA fallback.

Chat pane: message list via WS streaming (useSessionStream hook),
input bar, POST /api/sessions/:id/messages on submit, markdown
rendering via react-markdown + remark-gfm, inline tool-call display.

Diff pane: fetches GET /api/sessions/:id/pending, shows pending
changes with file path + operation badge (create/edit/delete),
before/after diff for edits, Approve/Reject per change and
Approve All/Reject All buttons.

Layout: fixed two-pane split (chat 60%, diff 40%). Dark theme
(bg-zinc-900). Desktop-first for v2.0.0.

Session picker (Home page): lists projects and sessions from the
shared DB. No CRUD — use BooChat's UI for that.

Dockerfile updated: builds web app in builder stage, copies dist
to runtime. index.ts registers fastifyStatic + SPA fallback route.

Tailwind v4, React 18, TypeScript strict. ~20 new files, ~370KB
built output. Functional developer tool UI, not polished consumer
product — Phase 7 (v2.0.3) handles polish.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/coder/src/index.ts

@@ -1,5 +1,12 @@
+import { resolve, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { existsSync } from 'node:fs';
 import Fastify from 'fastify';
 import fastifyWebsocket from '@fastify/websocket';
+import fastifyStatic from '@fastify/static';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
 import { loadConfig } from './config.js';
 import { getSql, applySchema, pingDb, closeDb } from './db.js';
 // v2.0.0 Phase 2B: workspace dependency on @boocode/server — reuse the
@@ -111,6 +118,28 @@ async function main() {
   registerPendingRoutes(app, sql);
   registerWebSocket(app, sql, broker);
 
+  // Serve static frontend (built web app). In production, the dist/ is
+  // copied to ../web relative to the dist/ directory at /app/web. In dev,
+  // check adjacent to the source.
+  const webRoot = resolve(__dirname, '../web');
+  if (existsSync(webRoot)) {
+    await app.register(fastifyStatic, {
+      root: webRoot,
+      prefix: '/',
+      // Don't intercept /api routes — static only serves files that exist.
+      wildcard: false,
+    });
+    // SPA fallback: serve index.html for non-API routes that don't match a file.
+    app.setNotFoundHandler(async (req, reply) => {
+      if (req.url.startsWith('/api')) {
+        reply.code(404);
+        return { error: 'not found' };
+      }
+      return reply.sendFile('index.html');
+    });
+    app.log.info(`serving frontend from ${webRoot}`);
+  }
+
   // Graceful shutdown
   const shutdown = async () => {
     app.log.info('shutting down');