feat: add --fast flag for quick startup (~15x faster)

[americanjeff]

• Add --fast command-line flag for instant initialization
• Standard mode (default): ~1600ms startup, waits for beep
• Fast mode: ~100ms startup, async beep with auto-trim
• Document both modes in README with usage examples
• Update CHANGELOG with new feature

Technical changes:

• Add #[derive(Clone)] to BeepPlayer for async spawning
• Conditional startup logic based on --fast flag
• Async beep playback with tokio::spawn in fast mode
• Maintains backward compatibility (standard mode is default)

Summary

Brief description of the changes in this PR.

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
• Performance improvement
• Code refactoring

Changes Made

• List the main changes
• Use bullet points
• Be specific about what was modified

Testing

• Tests pass locally with cargo test
• Code follows the project's style guidelines (cargo fmt and cargo clippy)
• Self-review of the code has been performed
• Code has been tested manually (if applicable)

Test Environment

• OS: [e.g. Arch Linux]
• Wayland Compositor: [e.g. Hyprland]
• Audio System: [e.g. PipeWire]

Audio Feedback Testing (if applicable)

• Recording start beep plays correctly
• Recording stop beep plays correctly
• Success beep plays after operations
• Error beep plays on failures
• Volume controls work as expected

Signal Testing (if applicable)

• SIGUSR1 (direct typing) works correctly
• SIGUSR2 (clipboard copy) works correctly
• Error handling works properly

Breaking Changes

If this PR contains breaking changes, please describe:

• What breaks?
• How to migrate existing usage?
• Updated documentation?

Additional Notes

Any additional information, context, or screenshots that would be helpful for reviewers.

Checklist

• My code follows the style guidelines of this project
• 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