feat(21-03): remove slide-out panels from root layout and clean UIStore

- Remove Item and Candidate SlideOutPanel instances from __root.tsx
- Remove SlideOutPanel, ItemForm, CandidateForm imports from root
- Remove panel state (panelMode, editingItemId, candidatePanelMode, editingCandidateId) from UIStore
- Remove panel actions (openEditPanel, openAddPanel, closePanel, etc.) from UIStore
- Preserve currentThreadId derivation for CandidateDeleteDialog
- Update CollectionView empty state to use catalog search instead of add panel
- Update ItemForm/CandidateForm with onClose prop replacing store panel close
- Clean all dead panel references across src/client/

src/client/stores/uiStore.ts

@@ -1,25 +1,15 @@
 import { create } from "zustand";
 
 interface UIState {
-	// Item panel state
-	panelMode: "closed" | "add" | "edit";
-	editingItemId: number | null;
+	// Item delete state
 	confirmDeleteItemId: number | null;
 
-	openAddPanel: () => void;
-	openEditPanel: (itemId: number) => void;
-	closePanel: () => void;
 	openConfirmDelete: (itemId: number) => void;
 	closeConfirmDelete: () => void;
 
-	// Candidate panel state
-	candidatePanelMode: "closed" | "add" | "edit";
-	editingCandidateId: number | null;
+	// Candidate delete state
 	confirmDeleteCandidateId: number | null;
 
-	openCandidateAddPanel: () => void;
-	openCandidateEditPanel: (id: number) => void;
-	closeCandidatePanel: () => void;
 	openConfirmDeleteCandidate: (id: number) => void;
 	closeConfirmDeleteCandidate: () => void;
 
@@ -70,28 +60,15 @@ interface UIState {
 }
 
 export const useUIStore = create<UIState>((set) => ({
-	// Item panel
-	panelMode: "closed",
-	editingItemId: null,
+	// Item delete
 	confirmDeleteItemId: null,
 
-	openAddPanel: () => set({ panelMode: "add", editingItemId: null }),
-	openEditPanel: (itemId) => set({ panelMode: "edit", editingItemId: itemId }),
-	closePanel: () => set({ panelMode: "closed", editingItemId: null }),
 	openConfirmDelete: (itemId) => set({ confirmDeleteItemId: itemId }),
 	closeConfirmDelete: () => set({ confirmDeleteItemId: null }),
 
-	// Candidate panel
-	candidatePanelMode: "closed",
-	editingCandidateId: null,
+	// Candidate delete
 	confirmDeleteCandidateId: null,
 
-	openCandidateAddPanel: () =>
-		set({ candidatePanelMode: "add", editingCandidateId: null }),
-	openCandidateEditPanel: (id) =>
-		set({ candidatePanelMode: "edit", editingCandidateId: id }),
-	closeCandidatePanel: () =>
-		set({ candidatePanelMode: "closed", editingCandidateId: null }),
 	openConfirmDeleteCandidate: (id) => set({ confirmDeleteCandidateId: id }),
 	closeConfirmDeleteCandidate: () => set({ confirmDeleteCandidateId: null }),