Fix appcast.xml to support both ARM64 and x86_64 via universal binary

[Copilot]

Problem

appcast.xml only contained x86_64 architecture entry, breaking Sparkle auto-updates for ARM64 (Apple Silicon) users.

Changes

Workflow: Create universal binary for Sparkle

• New step extracts both ARM64 and x86_64 builds
• Combines binaries using lipo -create
• Packages universal app as TablePro-{VERSION}.zip using ditto (preserves macOS metadata)
• Verifies both architectures present before packaging

Workflow: Update Sparkle appcast generation

• Changed from processing both TablePro-{VERSION}-arm64.zip and TablePro-{VERSION}-x86_64.zip
• Now generates appcast from single universal TablePro-{VERSION}.zip
• Results in single appcast item supporting both architectures

Result

Sparkle auto-updates now work for both ARM64 and x86_64 users via single universal binary. Architecture-specific DMGs/ZIPs remain available for manual download.

<!-- Before: Only x86_64 -->
<channel>
  <title>TablePro-x86_64</title>
  <item>
    <enclosure url=".../TablePro-0.1.1-x86_64.zip" ... />
  </item>
</channel>

<!-- After: Universal binary supporting both -->
<channel>
  <title>TablePro</title>
  <item>
    <enclosure url=".../TablePro-0.1.2.zip" ... />
  </item>
</channel>

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