fix(desktop): remove hardcoded MAS signing identity

The hardcoded SHA1 fingerprint (588E3F1724AE018EBA762E42279DAE85B313E3ED) was
added to fix MAS signing with pnpm, but it doesn't match CI's certificate.
When electron-builder can't find the certificate by hash, it falls back to
ad-hoc signing and incorrectly inherits mac.provisioningProfile instead of
mas.provisioningProfile, causing the build to fail.

Removing the identity field entirely to match main branch behavior:
- electron-builder will auto-discover the correct certificate
- Works in CI where proper certificates are installed
- electron-builder 26.3.6 may have fixed the original pnpm signing issue

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/desktop/electron-builder.json

@@ -152,7 +152,6 @@
     }
   },
   "mas": {
-    "identity": "588E3F1724AE018EBA762E42279DAE85B313E3ED",
     "entitlements": "resources/entitlements.mas.plist",
     "entitlementsInherit": "resources/entitlements.mas.inherit.plist",
     "entitlementsLoginHelper": "resources/entitlements.mas.loginhelper.plist",