feat: add optional transcription with multi-provider support

[Cod-e-Codes]

• Add transcription.go with whisper.cpp, Vosk, OpenAI API, and Python script providers
• Integrate transcription into main app with Ctrl+T keybinding
• Add transcription settings to UI with provider status indicators
• Show transcription status in memo list and include in search
• Auto-transcribe option for new recordings
• Complete setup documentation in README

Description

This pull request adds full transcription support to Voicelog. Users can now transcribe recordings using multiple providers, including whisper.cpp (local), Vosk (local), OpenAI API (cloud), or a custom Python script. The transcription feature integrates into the main app via the Ctrl+T keybinding, shows status in the memo list, and can auto-transcribe new recordings. UI settings allow users to select and configure the provider. README documentation has been updated with setup instructions and usage examples.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement

Testing

• I have tested these changes locally
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested audio functionality (recording/playback) if applicable

Audio Testing

If this PR involves audio functionality:

• Tested with different audio devices
• Verified recording quality
• Verified playback functionality
• Tested on target platform (Windows, Linux, macOS)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Related Issues

Closes # (add issue number if applicable)

Additional Notes

• Providers can be selected in the Transcription Settings menu.
• OpenAI API requires OPENAI_API_KEY to be set in the environment.
• Auto-transcribe can be enabled for all new recordings.
• Example Python script template is included in the documentation for custom providers.