Add prompt diff viewer and keyboard shortcuts

- DiffView: LCS-based unified message diff between any two calls
  Shows added/removed/shared messages with color-coded indicators,
  token and cost deltas, truncated shared messages for focus on changes
- Compare mode: toggle via filter bar button or 'c' key,
  select two calls to see side-by-side comparison
- Keyboard shortcuts: j/k navigate, Enter/o expand, / search,
  Esc close, c compare, t theme toggle
- Focus ring on keyboard-navigated call rows

Redesign UI: light-first warm minimal theme with dual mode support