[FEATURE] Implement read-only diff viewer for file changes

[UncleTallest]

Goal: Show what files Claude touched during conversation (read-only display).

Tasks:

• Detect file operations from conversation
• Parse file paths and changes
• Create diff display widget (before/after comparison)
• Syntax highlighting for code diffs
• Test with various file types
• Handle cases with no file changes

Success criteria:

• User can see what files Claude modified
• Diffs are readable with clear before/after
• Works for common file types (Python, JavaScript, etc.)

Output: src/claude_devel_client/ui/diff_viewer.py