fix: prevent silent auth error swallowing in fetch error utility

- Add setAuthErrorHandler() registration mechanism to utils/fetchError.ts
- Implement fallback logging when auth errors (401/403) occur without registered handler
- Update AuthProvider to register both API client and fetch error handlers
- Ensure auth errors are handled deterministically at multiple layers
- Add comprehensive tests for auth error handler registration and fallback logging
- Update Web-Development.md documentation with auth error handling contract

Fixes issue #21: Silent auth errors are now caught and logged if the handler is not
registered, preventing actionable errors from being silently swallowed.

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

Docs/Tasks.md

@@ -1,21 +1,3 @@
-## 20) Loading UX lacks progressive/skeleton states
-- Where found:
-	- [frontend/src/pages](frontend/src/pages)
-- Why this is needed:
-	- Blank loading regions reduce perceived responsiveness.
-- Goal:
-	- Introduce standardized loading placeholders.
-- What to do:
-	- Build shared skeleton components for tables/charts/forms.
-- Possible traps and issues:
-	- Skeleton mismatch with actual layout can cause content shift.
-- Docs changes needed:
-	- Add loading UX component guidance.
-- Doc references:
-	- [Docs/Web-Design.md](Docs/Web-Design.md)
-
----
-
 ## 21) Silent auth error swallow in fetch error utility
 - Where found:
 	- [frontend/src/utils/fetchError.ts](frontend/src/utils/fetchError.ts)