refactor: Modularize server into separate concerns

Split monolithic index.ts (~1400 lines) into modular structure:
- config.ts: Server configuration and constants
- db/: Database initialization, migrations, and seeds
- routes/: API handlers by domain (themes, canvas, components, etc.)
- services/: Terminal WebSocket server
- utils/: CORS helpers

Entry point now only coordinates initialization.

server/routes/config.ts

@@ -0,0 +1,27 @@
+import { db } from '../db'
+import { jsonResponse } from '../utils/cors'
+
+export async function handleConfig(req: Request, url: URL) {
+  if (req.method === 'GET') {
+    const key = url.searchParams.get('key')
+    if (key) {
+      const row = db.query('SELECT value FROM config WHERE key = ?').get(key) as { value: string } | null
+      return jsonResponse({ value: row?.value || null })
+    }
+    const rows = db.query('SELECT * FROM config').all()
+    return jsonResponse(rows)
+  }
+
+  if (req.method === 'POST') {
+    const body = await req.json()
+    const stmt = db.prepare('INSERT OR REPLACE INTO config (key, value) VALUES (?, ?)')
+    stmt.run(body.key, body.value)
+    return jsonResponse({ success: true })
+  }
+
+  return null
+}
+
+export function handleHealth() {
+  return jsonResponse({ status: 'ok', timestamp: new Date().toISOString() })
+}