feat: add configurable OSC 52 clipboard support

[sei40kr]

Summary

Adds OSC 52 terminal escape sequence support for clipboard operations as an opt-in configuration option.

Changes

• Created modular clipboard architecture with trait-based backends:
  • ArboardClipboard: System clipboard (default)
  • Osc52Clipboard: Terminal escape sequences
• Added use_osc52_clipboard config option (default: false)
• When enabled, uses OSC 52; when disabled, uses arboard
• Returns None if arboard initialization fails and OSC 52 is disabled

Benefits

• ✅ Enables clipboard functionality in SSH sessions
• ✅ Works in headless/remote environments
• ✅ No external clipboard tools required when using OSC 52
• ✅ Backward compatible - disabled by default

How to use

Users can test if their terminal supports OSC 52 by running:

printf '\033]52;c;%s\007' "$(echo -n 'test' | base64)"
# Then try pasting - if you see 'test', OSC 52 is supported

Then enable it in ~/.config/gitu/config.toml:

[general]
use_osc52_clipboard = true

Test plan

• All existing tests pass
• Builds successfully
• Default behavior unchanged (OSC 52 disabled by default)
• Config option properly documented in default_config.toml

🤖 Generated with Claude Code

[sei40kr]

Design Decision: Manual Configuration

OSC 52 support requires manual configuration rather than automatic detection. I explored auto-detection via XTGETTCAP queries, but encountered issues:

• Event loop interference: Background threads checking terminal capabilities conflicted with crossterm's event polling, causing menus to open unexpectedly at startup
• Synchronous checks: Running capability checks during copy operations would add 200ms latency to the first copy
• Terminal compatibility: Many terminals support OSC 52 but don't respond to XTGETTCAP queries (Windows Terminal, some tmux configurations)

Manual configuration via use_osc52_clipboard provides a reliable, zero-overhead solution.