fix: filter source control commits by branch

[chriswritescode-dev]

The commits tab in the source control dialog was showing commits from all branches (git log --all). Now it filters to the branch being viewed by passing the current branch through to the backend git log call.

The backend falls back to HEAD (instead of --all) when no branch is provided, and trims blank values to HEAD for safety.

Changes:

• Backend GitService.getLog accepts optional branch param, uses branch?.trim() || 'HEAD' instead of --all
• Backend route parses ?branch= query param and forwards it
• Frontend fetchGitLog/useGitLog accept branch param in query and key
• CommitsTab accepts branch prop and passes it through
• SourceControlPanel passes displayBranch to CommitsTab
• Tests added for explicit branch, blank fallback, and default HEAD

Validation:

• pnpm lint