feat(ui): Data Sources UI — enterprise browser for all 5 source databases

New "Data Sources UI" tab with per-database Browser (catalog + sample data),
Query Console (SqlWorkbench) and embedded interactive Shell (DbShell via SSH).

Backend:
- Extend sql_console.py with Cassandra (CQL) + Neo4j (Cypher) engines
- Add GET /api/sql/catalog/{engine} and GET /api/sql/sample/{engine}
- ssh_terminal: optional initial_command for auto-launching DB CLIs

Frontend:
- DataSourcesView with 5-DB rail, health dots, Browser/Console/Shell sub-tabs
- DbShell embedded xterm terminal with docker exec CLI per engine
- Deep-link topology DB nodes to Data Sources UI (no SQL dock on platform)
- WorkbenchPanel restricted to agent mode only — frees dashboard space

api/ssh_terminal.py

@@ -86,6 +86,13 @@ async def ssh_session(ws: WebSocket) -> None:
 
         await _send(ws, "connected", message="connected")
 
+        initial_cmd = cfg.get("initial_command")
+        if initial_cmd and chan and not chan.closed:
+            try:
+                chan.send(initial_cmd if initial_cmd.endswith("\n") else initial_cmd + "\n")
+            except Exception:
+                pass
+
         async def pump_out() -> None:
             assert chan is not None
             while True: