feat: server-first terminal creation, broadcast-only WS clients

- Add POST /create-terminal endpoint with MAX_TERMINALS=5 limit
- Server creates PTY, runs command, registers and broadcasts atomically
- Frontend startTerminal() calls server first, connects in reconnect mode
- Remove registerTerminalOnServer() — server handles registration
- Separate broadcast-only WS clients from PTY clients (no phantom "main" PTY)
- All broadcast functions use broadcastToAll() helper
- Fix resume existing flow to create terminal with --resume flag

server/config.ts

@@ -9,6 +9,7 @@ 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 = 10000
+export const MAX_TERMINALS = 5
 
 // Database
 export const DB_PATH = 'agent-ui.db'