fix: correct background bleed in code viewer and alignment in branch selector

Related Issue

N/A

Changes

• CodeViewer — Add min-w-max to the inner <pre> so the Shiki background extends to the full content width during horizontal scroll, removing the colored band on mobile.
• BranchSelector — Add [&>span]:min-w-0 flex-1 text-left to SelectTrigger so the line-clamped span can shrink inside the flex container, eliminating leading whitespace before long truncated branch names.

How to Verify

• Code viewer — Open a file with long lines on a narrow viewport, scroll horizontally, and confirm no background band appears in either theme.
• Branch selector — Select both a short and a long branch; both should start flush with the trigger's left padding (long names truncated with …).

Checklist

• npm run format:check passes
• npm run lint passes
• npm run typecheck passes
• npm run build passes
• PR title follows Conventional Commits (feat:, fix:, chore:, refactor:, docs:, test:, )