Initial MCM project: FastAPI + Textual TUI unified messenger

MultiCustomerMessenger supporting Telegram (python-telegram-bot),
WhatsApp (Green API) and SMS (python-gsmmodem-new). REST API with
Bearer-token auth, SQLAlchemy models for MariaDB, APScheduler for
background polling, and Textual TUI running in same asyncio event-loop.

tui/app.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+from textual.app import App
+
+from tui.screens.main_screen import MainScreen
+
+
+class MCMApp(App):
+    """MCM – MultiCustomerMessenger TUI."""
+
+    TITLE = "MCM – MultiCustomerMessenger"
+    CSS_PATH = "styles.tcss"
+    SCREENS = {"main": MainScreen}
+
+    def on_mount(self) -> None:
+        self.push_screen("main")