Git: branch switcher + pull/fetch

[Oaklight]

Problem

Currently only push works (on the current branch). No way to:

• See / switch branches
• Create a branch
• Pull or fetch from remote

Proposed

• Top of Git tab: dropdown showing current branch with list of local + remote branches
• Action buttons: 'New branch', 'Pull', 'Fetch'
• Warn before switching if there are unsaved or uncommitted changes

Notes

Backend extensions in git_ops.py; frontend in Git tab.

Priority: P2