preview: zen mode, native images, glow rendering

- Add --zen flag for distraction-free markdown viewing
- iTerm2/WezTerm native inline image protocol with tmux passthrough
- Improved chafa fallback with format detection
- Glow-based markdown rendering with custom theme
- Fix is_fzf() to check stdout TTY
- Enable tmux allow-passthrough for image protocol

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

home/.tmux.conf

@@ -20,6 +20,7 @@ set -g history-limit 100000
 set -sg escape-time 10
 set -g focus-events on
 set -g display-time 4000
+set -g allow-passthrough on
 
 # Mouse
 set -g mouse on