fix(linux): enable decorations=false for GTK HeaderBar dragging

Window manager decorations must be disabled for the GTK HeaderBar
to be draggable on Linux. The Overlay titleBarStyle remains for macOS.

Ref: task-264

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: pythoninthegrass

crates/mt-tauri/tauri.conf.json

@@ -18,6 +18,7 @@
         "height": 720,
         "resizable": true,
         "fullscreen": false,
+        "decorations": false,
         "minWidth": 800,
         "minHeight": 600,
         "titleBarStyle": "Overlay",