Add terminal graphics protocol support and fullscreen preview

[ignaciotcrespo]

Summary

• Add detection of terminal graphics protocols (Kitty, iTerm2, WezTerm, Ghostty) with automatic fallback to half-block rendering
• --screenshot now displays the captured image inline in the terminal using the native graphics protocol at full terminal size
• New f key in the TUI to view screenshots fullscreen — temporarily exits alt-screen, renders the image using the native protocol, and returns on any keypress

Details

Graphics protocols (Kitty/iTerm2) can't be used inside Bubbletea's alt-screen mode because the escape sequences break the panel layout. The approach taken:

• TUI panels: always use half-block (safe, works everywhere)
• Fullscreen mode (f): pauses Bubbletea via tea.Exec, exits alt-screen, renders with native protocol, waits for keypress, resumes TUI
• CLI --screenshot: renders inline using native protocol at full terminal size

Test plan

• Run --screenshot <preview> in Kitty/iTerm2/WezTerm — image should render at full terminal size with native quality
• Run --screenshot <preview> in a basic terminal — should fallback to half-block
• In TUI, capture a screenshot with s, then press f — should show fullscreen image, any key returns
• Verify f fullscreen appears in the help bar
• Verify panels are not broken in normal TUI mode