Feature: Show diff by default for branches/PRs

## Feature Request

Display the diff view by default when viewing branches or pull requests.

## Description

Users should see the code differences immediately when selecting a branch or PR, rather than having to manually open or toggle the diff view.

## Proposed Implementation

- Set diff view as the default display mode
- Auto-expand diff on branch/PR selection
- Maintain user preference for diff display settings
- Add option to remember last view state
- Include summary statistics (files changed, additions, deletions)

## UI/UX Considerations

- Collapsible diff sections for better navigation
- Syntax highlighting for code changes
- Side-by-side or unified diff view options
- File tree navigation for large diffs
- Quick jump to specific files
- Line numbers and change indicators (+/-)

## Configuration Options

- User preference to disable auto-show diff
- Choice of default diff view (unified vs split)
- Configurable diff context lines
- Option to hide/show whitespace changes
- File exclusion patterns

## Benefits

- Immediate visibility of changes
- Reduced clicks to view important information
- Faster code review workflow
- Better understanding of branch/PR scope

## Acceptance Criteria

- [ ] Diff is displayed by default when selecting branch/PR
- [ ] User can configure default behavior
- [ ] Diff view is performant for large changesets
- [ ] Navigation within diff is intuitive
- [ ] Settings are persisted across sessions

🤖 Generated with [Claude Code](https://claude.ai/code)

Start by locating the branch/PR selection entry point and the existing diff-view UI in the Electron app. Compare the current behavior with the acceptance criteria, then determine how preferences, large changesets, navigation, and view options should be handled; done means the configured diff view appears on selection and settings persist across sessions.

Written by the indexing model from the issue text.