Replace hardcoded dark-mode colors with theme-aware semantic classes
(bg-popover, text-foreground, bg-accent, etc.) so the branch picker
works in both light and dark mode. Detect the real default branch via
git symbolic-ref and show a "default" label (with "current" taking
precedence when both apply).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>