fix(windows): correct window title and remove extra titlebar spacing

- Add missing "title" field to tauri.windows.conf.json to display
  "CC Switch" instead of default "Tauri app"
- Make DRAG_BAR_HEIGHT platform-aware: 0px on Windows/Linux (native
  titlebar), 28px on macOS (Overlay mode needs traffic light space)
- Apply same fix to FullScreenPanel component for consistency

Fixes the issue where Windows showed wrong title and had ~28px extra
blank space below the native titlebar introduced in v3.9.0.

src-tauri/tauri.windows.conf.json

@@ -4,6 +4,7 @@
     "windows": [
       {
         "label": "main",
+        "title": "CC Switch",
         "titleBarStyle": "Visible",
         "minWidth": 900,
         "minHeight": 600