feat: TUI als Browser-App via textual serve

TUI von direkten Service-Importen auf httpx API-Calls umgestellt.
Neue tui/api_client.py als schlanker async HTTP-Client. Background-
Worker pollt API alle 5s für Echtzeit-Updates. Neuer tui_standalone.py
Entry-Point für 'textual serve --port 8001 tui_standalone.py'.

tui/app.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 from textual.app import App
 
+from tui.api_client import MCMApiClient
 from tui.screens.main_screen import MainScreen
 
 
@@ -12,5 +13,9 @@ class MCMApp(App):
     CSS_PATH = "styles.tcss"
     SCREENS = {"main": MainScreen}
 
+    def __init__(self) -> None:
+        super().__init__()
+        self._api_client = MCMApiClient()
+
     def on_mount(self) -> None:
         self.push_screen("main")