Add auto-hide vertical scrollbar to diff pane

[benvinegar]

Summary

Adds a vertical scrollbar to the diff pane with auto-hide behavior and drag-to-scroll functionality.

Features

• 1-character width scrollbar positioned on the right side of the diff pane
• Drag to scroll - click and drag the scrollbar thumb to navigate content
• Proportional thumb sizing - thumb height reflects the ratio of viewport to content (e.g., viewing 25% of content = thumb is 25% of track)
• Auto-hide behavior - scrollbar appears when scrolling (keyboard, mousewheel) and hides after 2 seconds of inactivity
• Hidden when content fits - scrollbar only appears when content exceeds viewport height
• Theme-aware styling - uses accentMuted color normally, accent color when dragging

Technical Implementation

• New VerticalScrollbar component in src/ui/components/scrollbar/
• Integrated into DiffPane with proper absolute positioning
• Scroll activity detection via existing 50ms polling mechanism
• Drag handling following existing PaneDivider patterns
• Total content height calculated from file section metrics

Testing

• All existing tests pass (191 tests)
• TypeScript typecheck passes
• Linting passes (oxlint + oxfmt)

Visual Preview

Active (content scrolls via keyboard/mouse):
┌─────────────────────────────────────────────────┬─┐
│ File content...                                 │█│
│ More content...                                 │█│  <- thumb visible
│ Even more...                                    │ │
│                                                 │ │
└─────────────────────────────────────────────────┴─┘

Idle (>2s):
┌─────────────────────────────────────────────────┐
│ File content...                                 │
│ More content...                                 │  <- scrollbar hidden
│ Even more...                                    │
│                                                 │
└─────────────────────────────────────────────────┘