fix(linux): repair unresponsive UI on startup and full-screen panels

Linux users reported the window UI (including native title bar buttons)
couldn't receive clicks until manually maximizing and restoring the
window. Root causes: (1) Tauri webview did not acquire focus on startup
so first clicks were consumed by X11/Wayland click-to-activate
(Tauri #10746, wry #637); (2) GTK surface input region failed to
renegotiate on the visible:false + show() path under some
WebKitGTK/compositor combinations.

- Add linux_fix::nudge_main_window helper that performs set_focus plus
  a ±1px no-op resize after window show, with a 500ms reconciliation
  readback to compensate for dropped resize requests on slow
  compositors.
- Wire the helper into every window re-show path: normal startup,
  deeplink, single_instance, tray show_main, and lightweight exit.
- Set WEBKIT_DISABLE_COMPOSITING_MODE=1 at startup to avoid resize
  crashes and Wayland surface negotiation issues.
- Remove data-tauri-drag-region on Linux from App.tsx header and the
  shared FullScreenPanel (used by all provider/MCP/workspace forms)
  to avoid Tauri #13440 in Wayland sessions. Extract drag-region
  constants to src/lib/platform.ts for reuse.

All Rust changes are gated by #[cfg(target_os = "linux")]; frontend
changes preserve macOS/Windows behavior via runtime isLinux() checks.
Known limitation: tiling Wayland compositors ignore set_size, so
GDK_BACKEND=x11 remains the user-side workaround.

src-tauri/src/lightweight.rs

@@ -36,6 +36,10 @@ pub fn exit_lightweight_mode(app: &tauri::AppHandle) -> Result<(), String> {
         let _ = window.unminimize();
         let _ = window.show();
         let _ = window.set_focus();
+        #[cfg(target_os = "linux")]
+        {
+            crate::linux_fix::nudge_main_window(window.clone());
+        }
         #[cfg(target_os = "windows")]
         {
             let _ = window.set_skip_taskbar(false);
@@ -66,6 +70,10 @@ pub fn exit_lightweight_mode(app: &tauri::AppHandle) -> Result<(), String> {
 
     if let Some(window) = app.get_webview_window("main") {
         let _ = window.set_focus();
+        #[cfg(target_os = "linux")]
+        {
+            crate::linux_fix::nudge_main_window(window.clone());
+        }
     }
 
     #[cfg(target_os = "windows")]