Add beta channel for early access releases

[JZimz]

Add a beta channel setting so users can opt-in to pre-release builds (rc, beta tags) and get early access to new features before stable releases.

This would let Discord community members test new features and catch bugs before stable ships. Since we're already marking pre-releases correctly in GitHub (v1.0.0-rc.1, etc.), this is just wiring up the client-side toggle.

Implementation:

• Add "Beta Channel" toggle in settings
• Update auto-updater to set allowPrerelease: true based on setting
• Requires app restart to switch channels
• Users on beta get both stable and pre-release updates

Post-v1.0 only - Keep it simple until we hit stable release.

References:

• GitHub Releases already distinguishes stable vs pre-release
• electron-updater supports this via allowPrerelease option
• Can add Canary (weekly builds) later if there's demand