Add Linux AppImage build target and app icon

The build config only had a Windows target. Adds a linux block producing
an AppImage, plus a 512x512 build/icon.png so the packaged app does not
ship with the stock Electron icon.

Verified by running the packaged binary: the embedded server comes up on
9095 and serves the overlay from process.resourcesPath, confirming the
extraResources mapping works in a packaged build.

StartupWMClass is deliberately not set via linux.desktop -- electron-builder
already emits the correct value, and the nested { entry: {...} } form leaks a
literal "entry=[object Object]" line into the .desktop file.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

desktop-client/package.json

@@ -26,7 +26,15 @@
       "sign": false,
       "signAndEditExecutable": false,
       "certificateSubjectName": null
-    }
+    },
+    "linux": {
+      "target": ["AppImage"],
+      "icon": "build/icon.png",
+      "category": "AudioVideo",
+      "synopsis": "YouTube Music player with a now-playing overlay for OBS",
+      "maintainer": "What's That Music"
+    },
+    "artifactName": "${productName}-${version}-${arch}.${ext}"
   },
   "dependencies": {
     "express": "^5.2.1",