feat: add branch switcher for code and commits views

Related Issue

N/A

Changes

• Add getBranches() to lib/github.ts; add ref param to getTree() and getContents(), sha param to getCommits() for branch-aware fetching
• Add BranchSelector client component (components/BranchSelector.tsx) using shadcn/ui Select — always opens downward, styled to match project theme
• Code tab: branch selector rendered in Sidebar header; file tree and code viewer reflect the selected branch
• Commits tab: branch selector rendered next to page heading; commit list filtered to selected branch; pagination URLs preserve ?branch=
• Pulls tab: no branch selector — PR list is repo-wide and branch-agnostic
• Branch state carried via ?branch=<name> query param; invalid or missing value falls back to default_branch
• Switching branch clears ?path to prevent 404 on files absent from the new branch
• Add --popover, --popover-foreground, --input, --ring CSS variables to app/globals.css required by shadcn/ui components

How to Verify

1. Open a repository page — branch selector visible in the sidebar header
2. Switch to a feature branch → file tree updates, ?path clears, URL shows ?branch=<name>
3. Select a file on the new branch → code viewer shows the correct content for that branch
4. Navigate to Commits tab → branch selector visible next to heading; commits are filtered to the selected branch
5. Navigate to Pulls tab → no branch selector shown; all PRs listed regardless of branch
6. Access ?branch=nonexistent → silently falls back to the default branch
7. Verify branch dropdown always opens downward and background is opaque in both light and dark mode

Checklist

• npm run build passes
• npm run lint passes
• New/updated strings added to both dictionaries/ko.json and dictionaries/en.json (if applicable)