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/components/common/FullScreenPanel.tsx

@@ -3,7 +3,12 @@ import { createPortal } from "react-dom";
 import { motion, AnimatePresence } from "framer-motion";
 import { ArrowLeft } from "lucide-react";
 import { Button } from "@/components/ui/button";
-import { isWindows, isLinux } from "@/lib/platform";
+import {
+  isWindows,
+  isLinux,
+  DRAG_REGION_ATTR,
+  DRAG_REGION_STYLE,
+} from "@/lib/platform";
 import { isTextEditableTarget } from "@/utils/domUtils";
 
 interface FullScreenPanelProps {
@@ -82,24 +87,27 @@ export const FullScreenPanel: React.FC<FullScreenPanelProps> = ({
           className="fixed inset-0 z-[60] flex flex-col"
           style={{ backgroundColor: "hsl(var(--background))" }}
         >
-          {/* Drag region - match App.tsx */}
-          <div
-            data-tauri-drag-region
-            style={
-              {
-                WebkitAppRegion: "drag",
-                height: DRAG_BAR_HEIGHT,
-              } as React.CSSProperties
-            }
-          />
+          {/* Drag region - match App.tsx. Linux 上 DRAG_BAR_HEIGHT=0，
+              直接跳过整个元素；macOS 保留 28px 拖拽占位。 */}
+          {DRAG_BAR_HEIGHT > 0 && (
+            <div
+              data-tauri-drag-region
+              style={
+                {
+                  WebkitAppRegion: "drag",
+                  height: DRAG_BAR_HEIGHT,
+                } as React.CSSProperties
+              }
+            />
+          )}
 
           {/* Header - match App.tsx */}
           <div
             className="flex-shrink-0 flex items-center"
-            data-tauri-drag-region
+            {...DRAG_REGION_ATTR}
             style={
               {
-                WebkitAppRegion: "drag",
+                ...DRAG_REGION_STYLE,
                 backgroundColor: "hsl(var(--background))",
                 height: HEADER_HEIGHT,
               } as React.CSSProperties
@@ -107,8 +115,8 @@ export const FullScreenPanel: React.FC<FullScreenPanelProps> = ({
           >
             <div
               className="px-6 w-full flex items-center gap-4"
-              data-tauri-drag-region
-              style={{ WebkitAppRegion: "drag" } as React.CSSProperties}
+              {...DRAG_REGION_ATTR}
+              style={{ ...DRAG_REGION_STYLE } as React.CSSProperties}
             >
               <Button
                 type="button"