Refactor: Extract branch loading logic into shared function

[built-by-as]

Summary

• Extracted duplicated git branch loading logic into a shared loadAndPopulateBranches function
• Reduced code duplication across directory selection handlers

Changes

• Created loadAndPopulateBranches helper function in renderer.ts (line 219)
• Refactored browse directory button handler to use the shared function
• Refactored new session modal initialization to use the shared function

Benefits

• Eliminates ~30 lines of duplicated code
• Improves maintainability by centralizing branch loading logic
• Consistent behavior across both directory selection flows
• Optional parameter supports pre-selecting a specific branch

Test plan

• Verify browsing for a directory loads branches correctly
• Verify opening new session modal loads branches from last settings
• Verify selected branch is properly pre-selected when opening the modal
• Verify "No git repository found" message displays when appropriate