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/config.ts

@@ -0,0 +1,13 @@
+// Server configuration
+export const PORT_HTTP = 4101
+export const PORT_TERMINAL = 4103
+
+// Terminal configuration
+export const WORKING_DIR = process.cwd().replace(/[\\\/]server$/, '')
+export const SHELL = process.platform === 'win32' ? 'powershell.exe' : 'bash'
+export const SHELL_ARGS = process.platform === 'win32' ? ['-NoLogo', '-NoProfile'] : []
+export const DEFAULT_SESSION_ID = 'main'
+export const MAX_BUFFER_LINES = 1000
+
+// Database
+export const DB_PATH = 'agent-ui.db'