feat(12-01): wire compare toggle and ComparisonTable into thread detail

- 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

src/client/stores/uiStore.ts

@@ -50,8 +50,8 @@ interface UIState {
 	closeExternalLink: () => void;
 
 	// Candidate view mode
-	candidateViewMode: "list" | "grid";
-	setCandidateViewMode: (mode: "list" | "grid") => void;
+	candidateViewMode: "list" | "grid" | "compare";
+	setCandidateViewMode: (mode: "list" | "grid" | "compare") => void;
 }
 
 export const useUIStore = create<UIState>((set) => ({