fix(desktop): crash on launch + wrong icon (deb)

- productName 'Fabric Desktop' put the app in /opt/Fabric Desktop/;
  the space broke Electron's zygote/chrome-sandbox execvp
  (failed to execvp: /opt/Fabric -> FATAL zygote_host, instant
  crash). productName -> 'Fabric' (/opt/Fabric, no space);
  linux.executableName=fabric-desktop, linux.desktop.Name keeps the
  'Fabric Desktop' label.
- Icon landed in hicolor/0x0/apps (theme ignores it -> generic gear).
  Ship a real build/icons/<NxN>.png set and point linux.icon there;
  deb now installs 16..1024 hicolor sizes.

Verified in the rebuilt .deb: /opt/Fabric/fabric-desktop, hicolor
16x16..1024x1024, no zygote core dump on run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -24,7 +24,7 @@
   },
   "build": {
     "appId": "ai.hangman.fabric.desktop",
-    "productName": "Fabric Desktop",
+    "productName": "Fabric",
     "artifactName": "Fabric-Desktop-${version}-${os}-${arch}.${ext}",
     "directories": {
       "output": "dist"
@@ -45,7 +45,13 @@
         "tar.gz"
       ],
       "category": "Utility",
-      "maintainer": "Hangman <noreply@hangman-lab.top>"
+      "maintainer": "Hangman <noreply@hangman-lab.top>",
+      "executableName": "fabric-desktop",
+      "icon": "build/icons",
+      "desktop": {
+        "Name": "Fabric Desktop",
+        "StartupWMClass": "Fabric Desktop"
+      }
     },
     "mac": {
       "target": [