feat: replace X11 clipboard with OSC 52 for better remote terminal support

[guox18]

Summary

This PR replaces the X11-based clipboard implementation with OSC 52 escape sequences.

Motivation

When using jless over SSH without X11 forwarding, the clipboard functionality fails with:

Unable to access clipboard: XCB connection error: ClosedParseErr

OSC 52 is a terminal escape sequence that allows copying to the local clipboard through the terminal emulator, without requiring X11. It's supported by most modern terminals including:

• iTerm2
• kitty
• Alacritty
• Windows Terminal
• tmux (with set -g set-clipboard on)

Changes

• Remove clipboard crate dependency (X11-based)
• Add base64 crate for encoding
• Implement OSC 52 escape sequence for clipboard operations
• The y commands now work in SSH sessions without X11 forwarding

Testing

Tested on remote Linux server via SSH, clipboard copy works correctly with iTerm2.

[thomassa]

The change in this branch looks as if it will remove the ability to copy to the clipboard, for people using X11 and a terminal emulator that does not support OSC 52.

For example, according to the tmux wiki:

• Non-default configuration is required for xterm and Kitty to support OSC 52.
• "rxvt-unicode does not support OSC 52 natively. There is an unofficial Perl extension here."
• "VTE terminals (GNOME terminal, XFCE terminal, Terminator) do not support the OSC 52 escape sequence."
  • I see that this is due to concerns about the security implications of allowing a remote server to read/change the contents of the local clipboard: there is a vte issue ticket with discussion.

Would it be practical to make jless use whichever is available, trying the pre-existing mechanism (from the clipboard crate) and falling back to OSC 52 on failure?

See also the approach taken in #121.