refactor(proxy): remove global auto-start flag

- Remove global proxy auto-start flag from config and UI.
- Simplify per-app takeover start/stop and stop server when the last takeover is disabled.
- Restore live takeover detection used for crash recovery.
- Keep proxy_config.enabled column but always write 0 for compatibility.
- Tests: not run (not requested).

src/types/proxy.ts

@@ -1,5 +1,4 @@
 export interface ProxyConfig {
-  enabled: boolean;
   listen_address: string;
   listen_port: number;
   max_retries: number;