- Extend uiStore candidateViewMode union to include "compare" value
- Add columns-3 compare toggle button, shown only when thread has 2+ candidates
- Hide "Add Candidate" button when in compare view (read-only intent)
- Import and render ComparisonTable when candidateViewMode === "compare"
- Pass displayItems so compare view reflects any pending reorder state
- Existing list/grid views unchanged; all 135 tests pass