Make the window a fixed shell with no scrollbars

Lock html/body/#root to the viewport and hide scrollbars everywhere; the
content area fills its space instead of scrolling the page, and the logs
stream scrolls within its own pane. Add a min window size so the layout
can't be shrunk below where everything fits.

src-tauri/tauri.conf.json

@@ -15,6 +15,8 @@
         "title": "TutaBridge",
         "width": 700,
         "height": 520,
+        "minWidth": 700,
+        "minHeight": 520,
         "resizable": true,
         "center": true
       }