feat: add HEAD option to branch selector dropdown

[TomKalina]

Summary

• Adds a dynamic HEAD (branch-name) option to the branch selector dropdown
• Automatically resolves to the currently checked-out branch when loading commits
• Persists across branch switches — the graph always follows the current HEAD without manual reselection

Changes

• web/utils.ts: Added HEAD_BRANCH constant
• web/main.ts:
  • getBranchOptions(): Adds "HEAD (branch-name)" option when HEAD is available
  • resolveHeadBranches(): Resolves HEAD to actual branch name before sending to backend
  • loadRepoInfo(): Preserves HEAD selection when filtering stale branches

Test plan

• Open Git Graph, verify "HEAD (branch-name)" appears in the branch dropdown
• Select HEAD, verify the graph shows commits for the current branch
• Switch branches via git checkout, reload graph, verify HEAD follows the new branch
• Select HEAD alongside other branches, verify both are shown

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