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/lib/platform.ts

@@ -29,3 +29,20 @@ export const isLinux = (): boolean => {
     return false;
   }
 };
+
+// Linux 上禁用所有 drag region，规避 Wayland 下 gtk_window_begin_move_drag
+// 相关的窗口事件异常（Tauri #13440）。macOS 上保留原有拖动行为；Windows
+// 项目原本就不依赖这个。
+//
+// 这些常量设计为通过 JSX 属性 spread 消费（`{...DRAG_REGION_ATTR}`），
+// 因为 `data-tauri-drag-region` 是 wry 侧的 attribute 存在性检测，必须
+// 完全不渲染属性才算禁用；空字符串或 "false" 仍会触发。
+export const DRAG_REGION_ENABLED = !isLinux();
+
+export const DRAG_REGION_ATTR: Record<string, unknown> = DRAG_REGION_ENABLED
+  ? { "data-tauri-drag-region": true }
+  : {};
+
+export const DRAG_REGION_STYLE: Record<string, unknown> = DRAG_REGION_ENABLED
+  ? { WebkitAppRegion: "drag" }
+  : {};