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/App.tsx

@@ -40,7 +40,12 @@ import { useLastValidValue } from "@/hooks/useLastValidValue";
 import { extractErrorMessage } from "@/utils/errorUtils";
 import { isTextEditableTarget } from "@/utils/domUtils";
 import { cn } from "@/lib/utils";
-import { isWindows, isLinux } from "@/lib/platform";
+import {
+  isWindows,
+  isLinux,
+  DRAG_REGION_ATTR,
+  DRAG_REGION_STYLE,
+} from "@/lib/platform";
 import { AppSwitcher } from "@/components/AppSwitcher";
 import { ProviderList } from "@/components/providers/ProviderList";
 import { AddProviderDialog } from "@/components/providers/AddProviderDialog";
@@ -876,11 +881,13 @@ function App() {
       className="flex flex-col h-screen overflow-hidden bg-background text-foreground selection:bg-primary/30"
       style={{ overflowX: "hidden", paddingTop: CONTENT_TOP_OFFSET }}
     >
-      <div
-        className="fixed top-0 left-0 right-0 z-[60]"
-        data-tauri-drag-region
-        style={{ WebkitAppRegion: "drag", height: DRAG_BAR_HEIGHT } as any}
-      />
+      {DRAG_BAR_HEIGHT > 0 && (
+        <div
+          className="fixed top-0 left-0 right-0 z-[60]"
+          data-tauri-drag-region
+          style={{ WebkitAppRegion: "drag", height: DRAG_BAR_HEIGHT } as any}
+        />
+      )}
       {showEnvBanner && envConflicts.length > 0 && (
         <EnvWarningBanner
           conflicts={envConflicts}
@@ -908,10 +915,10 @@ function App() {
 
       <header
         className="fixed z-50 w-full transition-all duration-300 bg-background/80 backdrop-blur-md"
-        data-tauri-drag-region
+        {...DRAG_REGION_ATTR}
         style={
           {
-            WebkitAppRegion: "drag",
+            ...DRAG_REGION_STYLE,
             top: DRAG_BAR_HEIGHT,
             height: HEADER_HEIGHT,
           } as any
@@ -919,8 +926,8 @@ function App() {
       >
         <div
           className="flex h-full items-center justify-between gap-2 px-6"
-          data-tauri-drag-region
-          style={{ WebkitAppRegion: "drag" } as any}
+          {...DRAG_REGION_ATTR}
+          style={{ ...DRAG_REGION_STYLE } as any}
         >
           <div
             className="flex items-center gap-1"