🤖 Fix: Restore base64: prefix for CSC_LINK (#75)

## Problem

PR #74 removed the `base64:` prefix from `CSC_LINK`, but
electron-builder requires this prefix to distinguish between:
- A file path to a certificate file
- Inline base64-encoded certificate data

Without the prefix, electron-builder treats the base64 string as a file
path and fails to find the certificate.

## Solution

Restore the `base64:` prefix:
```yaml
CSC_LINK: base64:${{ secrets.MACOS_CERTIFICATE }}
```

## How it works

- `MACOS_CERTIFICATE` secret contains the raw base64-encoded .p12
certificate
- The `base64:` prefix tells electron-builder to decode the certificate
data inline
- Electron-builder creates a temporary keychain and imports the
certificate automatically

## References

- Codex comment on PR #74 identifying the issue
- electron-builder documentation on CSC_LINK format

_Generated with `cmux`_

Author: ammario

.github/workflows/build.yml

@@ -28,7 +28,7 @@ jobs:
 
       - name: Package for macOS
         env:
-          CSC_LINK: ${{ secrets.MACOS_CERTIFICATE }}
+          CSC_LINK: ${{ secrets.MACOS_CERTIFICATE && format('base64:{0}', secrets.MACOS_CERTIFICATE) || '' }}
           CSC_KEY_PASSWORD: ${{ secrets.MACOS_CERTIFICATE_PWD }}
         run: bun run dist:mac