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/main.rs

@@ -10,6 +10,12 @@ fn main() {
         if std::env::var("WEBKIT_DISABLE_DMABUF_RENDERER").is_err() {
             std::env::set_var("WEBKIT_DISABLE_DMABUF_RENDERER", "1");
         }
+        // 禁用 WebKitGTK 合成模式，规避 resize 时 webview 崩溃以及部分 Wayland
+        // 合成器下的 surface 协商问题（整窗 UI 点击无响应、必须最大化-还原才能恢复）。
+        // 参考: https://github.com/tauri-apps/tauri/issues/9394
+        if std::env::var("WEBKIT_DISABLE_COMPOSITING_MODE").is_err() {
+            std::env::set_var("WEBKIT_DISABLE_COMPOSITING_MODE", "1");
+        }
     }
 
     cc_switch_lib::run();