fix(gui): resolve GNOME taskbar icon via WM_CLASS/StartupWMClass match

Window/taskbar icon never showed because the packaged .desktop had
StartupWMClass=ModelRouter while the live window WM_CLASS is the lowercase
app class 'modelrouter-gui' — no match, so GNOME fell back to the default
Electron icon.

- main.js: set BrowserWindow icon asset + backgroundColor (window paints
  correctly with frameless titlebar and provides the taskbar's default icon)
- package.json: build.linux.desktop.entry.StartupWMClass now equals the real
  WM_CLASS so packaged installs match too
- local integration: user-level .desktop installed at
  ~/.local/share/applications/modelrouter-gui.desktop (points at the unpacked
  binary) so GNOME resolves the icon immediately

cmd/gui/package.json

@@ -22,7 +22,7 @@
   "build": {
     "appId": "com.modelrouter.gui",
     "productName": "ModelRouter",
-    "linux": {
+"linux": {
       "target": [
         "deb",
         "rpm",
@@ -32,6 +32,11 @@
       "icon": "build/icon.png",
       "maintainer": "ModelRouter",
       "synopsis": "ModelRouter Desktop",
+"desktop": {
+        "entry": {
+            "StartupWMClass": "modelrouter-gui"
+        }
+      },
       "extraResources": [
         {
           "from": "bin/llmsproxy",