Add skeleton loading components for progressive UX

Implement standardized skeleton loading placeholders to reduce perceived
loading time and prevent layout shift during data fetches. These components
match actual content dimensions exactly, improving perceived responsiveness.

New skeleton components in src/components/skeletons/:
- SkeletonTable: Table/grid loading with customizable rows and cells
- SkeletonTableRow: Individual animated skeleton row
- SkeletonChart: Chart/graph loading with bars matching dimensions
- SkeletonStat: Stat card loading with label and value
- SkeletonFormField: Form input loading placeholder
- PageLoadingSkeleton: Convenience wrapper for page-level loading states

Implementation details:
- All skeletons use global 'skeleton-pulse' animation (2s cycle)
- Dimensions match real content to prevent layout shift on arrival
- Marked with aria-hidden and role=presentation for accessibility
- Theme-aware colors using Fluent UI tokens
- Respects prefers-reduced-motion setting

Updates:
- ChartStateWrapper: Uses SkeletonChart instead of spinner
- PageFeedback: Added PageLoadingSkeleton component
- App.tsx: Injects skeleton styles at startup
- Web-Design.md: Added § 8a with loading UX guidance and usage examples

All components tested (22 tests, 100% passing) and linted.

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

Docs/Tasks.md

@@ -1,22 +1,3 @@
-## 19) Provider dependency chain is implicit
-- Where found:
-	- [frontend/src/App.tsx](frontend/src/App.tsx)
-- Why this is needed:
-	- Order-sensitive providers can fail silently during future refactors.
-- Goal:
-	- Make provider dependency order explicit and tested.
-- What to do:
-	- Document provider order rationale.
-	- Add provider composition tests.
-- Possible traps and issues:
-	- Hidden assumptions in auth/timezone/theme initialization.
-- Docs changes needed:
-	- Add provider order contract.
-- Doc references:
-	- [Docs/Web-Development.md](Docs/Web-Development.md)
-
----
-
 ## 20) Loading UX lacks progressive/skeleton states
 - Where found:
 	- [frontend/src/pages](frontend/src/pages)