feat: add direct .tableplugin bundle installation

[datlechin]

Summary

• Add three new install paths for .tableplugin bundles: file picker selection, Finder double-click, and drag-and-drop onto the Plugins settings view
• Register com.tablepro.plugin UTType so macOS associates .tableplugin files with TablePro
• Refactor PluginManager.installPlugin(from:) to route by file extension (.tableplugin vs .zip)

Test plan

• File picker shows both .zip and .tableplugin files
• Selecting a .tableplugin bundle installs it correctly
• Selecting a .zip still works as before
• Drag .tableplugin onto Plugins settings view installs the plugin
• Double-click .tableplugin in Finder opens TablePro, installs, and navigates to Plugins tab
• Installing a plugin with a conflicting built-in ID shows error
• Invalid/unsigned bundles are rejected with appropriate error

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