Consolidate frontend storage keys into constants module

- Move magic strings from AuthProvider, MainLayout, and ThemeProvider to
  frontend/src/utils/constants.ts
- Add STORAGE_KEY_AUTHENTICATED, STORAGE_KEY_SIDEBAR_COLLAPSED, and
  STORAGE_KEY_THEME constants with JSDoc descriptions
- Update all three files to import and use centralized keys
- Prevents key drift and typo regressions across the frontend

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/utils/constants.ts

@@ -3,6 +3,10 @@
  */
 import type { BanOriginFilter, TimeRange } from "../types/ban";
 
+// ---------------------------------------------------------------------------
+// UI Constants
+// ---------------------------------------------------------------------------
+
 export const BAN_ORIGIN_FILTER_LABELS: Record<BanOriginFilter, string> = {
   all: "All",
   blocklist: "Blocklist",
@@ -21,3 +25,16 @@ export const BAN_PAGE_SIZE = 100 as const;
 
 /** Items per page for the history table. */
 export const HISTORY_PAGE_SIZE = 50 as const;
+
+// ---------------------------------------------------------------------------
+// Storage Keys
+// ---------------------------------------------------------------------------
+
+/** SessionStorage key for authentication state persistence. */
+export const STORAGE_KEY_AUTHENTICATED = "bangui_authenticated" as const;
+
+/** LocalStorage key for sidebar collapsed state. */
+export const STORAGE_KEY_SIDEBAR_COLLAPSED = "bangui_sidebar_collapsed" as const;
+
+/** LocalStorage key for theme preference. */
+export const STORAGE_KEY_THEME = "bangui_theme" as const;