Fix compilation errors in email and owhisper-server crates

[devin-ai-integration]

Fix compilation errors in email and owhisper-server crates

Summary

This PR fixes two compilation errors discovered during dependency analysis:

1. email crate: Added conditional compilation attributes for macOS-only objc2 imports. The crate was failing to compile on Linux because it unconditionally imported objc2 modules that are only available on macOS. Added a stub implementation for non-macOS platforms that returns an error message.

2. owhisper-server crate: Fixed incorrect type reference from hypr_agc::Agc to hypr_agc::VadAgc in the realtime audio processing code. The agc crate exports VadAgc, not Agc.

3. schema.json: Auto-generated alphabetical reordering of JSON schema properties (side effect of build process).

Review & Testing Checklist for Human

• Verify email functionality on macOS: Test that email composition still works correctly on macOS (the primary platform for this feature)
• Test owhisper-server realtime mode: Verify that the realtime audio processing with AGC still functions correctly after the type name fix
• Confirm schema.json changes are expected: The alphabetical reordering appears to be auto-generated, but verify this is intentional

Test Plan

1. On macOS: Try using the email composition feature to ensure objc2 integration still works
2. Run owhisper-server in realtime mode and verify audio processing with AGC works correctly
3. Build the project on Linux/Windows to confirm the email crate compiles successfully

Notes

• These fixes were discovered while checking for unused dependencies using cargo-machete
• Both crates now compile successfully with cargo check --all-targets
• The email crate stub for non-macOS platforms returns an error, which is appropriate since the feature is macOS-only
• Session: https://app.devin.ai/sessions/ebe1f1db1fd246968b338cece40099a7
• Requested by: yujonglee (@yujonglee)

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