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>

Docs/Tasks.md

@@ -1,21 +1,3 @@
-## 21) Silent auth error swallow in fetch error utility
-- Where found:
-	- [frontend/src/utils/fetchError.ts](frontend/src/utils/fetchError.ts)
-- Why this is needed:
-	- Silent return can drop actionable errors when no auth handler is registered.
-- Goal:
-	- Ensure auth errors are handled deterministically with fallback logging/handling.
-- What to do:
-	- Enforce handler registration or fallback behavior.
-- Possible traps and issues:
-	- Duplicate redirect flows if multiple auth handlers exist.
-- Docs changes needed:
-	- Add auth error handling contract for utilities.
-- Doc references:
-	- [frontend/src/providers/AuthProvider.tsx](frontend/src/providers/AuthProvider.tsx)
-
----
-
 ## 22) Magic strings are scattered in frontend storage keys
 - Where found:
 	- [frontend/src/providers/AuthProvider.tsx](frontend/src/providers/AuthProvider.tsx)