fix: improve diff viewer render and hunk navigation performance - #16

What changed

• Removed full-screen clearing on every terminal redraw.
• Cached normalized file lines during diff loading for reuse while rendering.
• Precomputed hunk starts and ranges per file instead of rebuilding them during navigation.

Why

• Reduce repeated work during scrolling, hunk navigation, and redraws.
• Let ratatui use incremental rendering more effectively.
• Improve responsiveness on larger diffs and files.

User-facing impact

• The UI should feel smoother when scrolling or navigating hunks.
• No intended behavior changes.

Validation

• cargo check --locked
• Manual testing completed (if applicable)

Checklist

• PR is focused and scoped to one change
• Docs updated when behavior changes
• Breaking changes called out (if any)