feat(web): migrate Items page to React with UI improvements

Phase 2 of frontend migration (epic #6, issue #8).

Rebuild the Items page (4,243 lines of vanilla JS) as 16 React
components with full feature parity plus UI improvements.

UI improvements:
- Footer stats bar (28px fixed bottom) replacing top stat cards
- Compact row density (28-32px) with alternating background colors
- Right-click column configuration via reusable ContextMenu component
- Resizable horizontal/vertical split panel layout (persisted)
- In-pane CRUD forms replacing modal dialogs (Infor-style)

Components (web/src/components/items/):
- ItemTable: sortable columns, alternating rows, column config
- ItemsToolbar: search with scope (All/PN/Desc), filters, actions
- SplitPanel: drag-resizable horizontal/vertical container
- FooterStats: fixed bottom bar with reactive item counts
- ItemDetail: 5-tab detail pane (Main, Properties, Revisions, BOM,
  Where Used) with header actions
- MainTab: metadata, inline project tag editor, file download
- PropertiesTab: form/JSON dual-mode editor, save as new revision
- RevisionsTab: comparison diff, status management, rollback
- BOMTab: inline CRUD, cost calculations, CSV export
- WhereUsedTab: parent assemblies table
- CreateItemPane: in-pane form with schema category properties
- EditItemPane: in-pane edit form for basic fields
- DeleteItemPane: in-pane confirmation with warning
- ImportItemsPane: CSV upload with dry-run validation flow

Shared components:
- ContextMenu: positioned right-click menu with checkbox support

Hooks:
- useItems: items fetching with search, filters, pagination, debounce
- useLocalStorage: typed localStorage state hook

Extended api/types.ts with request/response types for search, BOM,
revisions, CSV import, schema properties, and revision comparison.

web/src/api/types.ts

@@ -3,7 +3,7 @@ export interface User {
   username: string;
   display_name: string;
   email: string;
-  role: 'admin' | 'editor' | 'viewer';
+  role: "admin" | "editor" | "viewer";
   auth_source: string;
 }
 
@@ -127,3 +127,104 @@ export interface ErrorResponse {
   error: string;
   message?: string;
 }
+
+// Search
+export interface FuzzyResult extends Item {
+  score: number;
+}
+
+// Where Used
+export interface WhereUsedEntry {
+  id: string;
+  parent_part_number: string;
+  parent_description: string;
+  rel_type: string;
+  quantity: number | null;
+  unit?: string;
+  reference_designators?: string[];
+}
+
+// CSV Import
+export interface CSVImportResult {
+  total_rows: number;
+  success_count: number;
+  error_count: number;
+  errors?: CSVImportError[];
+  created_items?: string[];
+}
+
+export interface CSVImportError {
+  row: number;
+  field?: string;
+  message: string;
+}
+
+// Request types
+export interface CreateItemRequest {
+  schema?: string;
+  category: string;
+  description: string;
+  projects?: string[];
+  properties?: Record<string, unknown>;
+  sourcing_type?: string;
+  sourcing_link?: string;
+  long_description?: string;
+  standard_cost?: number;
+}
+
+export interface UpdateItemRequest {
+  part_number?: string;
+  item_type?: string;
+  description?: string;
+  properties?: Record<string, unknown>;
+  comment?: string;
+  sourcing_type?: string;
+  sourcing_link?: string;
+  long_description?: string;
+  standard_cost?: number;
+}
+
+export interface CreateRevisionRequest {
+  properties: Record<string, unknown>;
+  comment: string;
+}
+
+export interface AddBOMEntryRequest {
+  child_part_number: string;
+  rel_type?: string;
+  quantity?: number;
+  unit?: string;
+  reference_designators?: string[];
+  child_revision?: number;
+  metadata?: Record<string, unknown>;
+}
+
+export interface UpdateBOMEntryRequest {
+  rel_type?: string;
+  quantity?: number;
+  unit?: string;
+  reference_designators?: string[];
+  child_revision?: number;
+  metadata?: Record<string, unknown>;
+}
+
+// Schema properties
+export interface PropertyDef {
+  type: string;
+  required?: boolean;
+  description?: string;
+  default?: unknown;
+}
+
+export type PropertySchema = Record<string, PropertyDef>;
+
+// Revision comparison
+export interface RevisionComparison {
+  from: number;
+  to: number;
+  added: Record<string, unknown>;
+  removed: Record<string, unknown>;
+  changed: Record<string, { from: unknown; to: unknown }>;
+  status_changed?: { from: string; to: string };
+  file_changed?: boolean;
+}