Add a SystemClock component that displays the current date and time in
British format. The clock appears in the top control bar (same row as
Clipboard and Actions buttons) and updates every second.
Features:
- Date format: "7 February 2026" (British English)
- Time format: "14:35:42" (24-hour)
- Auto-updates every second via setInterval
- Proper cleanup on unmount using Svelte 5 $effect
- Hover effect with accent colour border
- Positioned with margin-left: auto to align right
Testing:
- Added comprehensive unit tests for date/time formatting logic
- 12 test cases covering edge cases, month boundaries, leap years
- All tests passing with proper local timezone handling
- Updated CLAUDE.md with testing requirements and guidelines
Resolves: #128
