feat: add shimmer loading skeletons for various pages and components

- Implemented UiShimmer as a core shimmer wrapper for animated gradient effects.
- Created shimmer presets for list items, stats cards, section headers, and more.
- Developed specific skeletons for billing, invoices, coverage, hubs, reports, payments, shifts, and home pages.
- Enhanced user experience by providing visual placeholders during data loading.

.claude/agent-memory/mobile-builder/MEMORY.md

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+# Mobile Builder Agent Memory
+
+## Design System - Shimmer Primitives
+- Shimmer widgets are in `packages/design_system/lib/src/widgets/shimmer/`
+- Available: `UiShimmer`, `UiShimmerBox`, `UiShimmerCircle`, `UiShimmerLine`, `UiShimmerListItem`, `UiShimmerStatsCard`, `UiShimmerSectionHeader`, `UiShimmerList`
+- `UiShimmerList.itemBuilder` takes `(int index)` -- single parameter, not `(BuildContext, int)`
+- `UiShimmerBox.borderRadius` accepts `BorderRadius?` (nullable), uses `UiConstants.radiusMd` as default
+- All shimmer shapes render as solid white containers; the parent `UiShimmer` applies the animated gradient
+- Exported via `design_system.dart` barrel
+
+## Staff App Feature Locations
+- Shifts: `packages/features/staff/shifts/` -- has ShiftsPage (tabbed: MyShifts/Find/History) + ShiftDetailsPage
+- Home: `packages/features/staff/home/` -- WorkerHomePage with sections (TodaysShifts, TomorrowsShifts, Recommended, Benefits, QuickActions)
+- Payments: `packages/features/staff/payments/` -- PaymentsPage with gradient header + stats + payment history
+- Home cubit: `HomeStatus` enum (initial, loading, loaded, error)
+- Shifts bloc: `ShiftsStatus` enum + sub-loading flags (`availableLoading`, `historyLoading`)
+- Payments bloc: uses sealed state classes (`PaymentsLoading`, `PaymentsLoaded`, `PaymentsError`)
+
+## UiConstants Spacing Tokens
+- Use `UiConstants.space1` through `UiConstants.space24` for spacing
+- Radius: `UiConstants.radiusSm`, `radiusMd`, `radiusLg`, `radiusFull`, `radiusBase`, `radiusMdValue` (double)
+- `UiConstants.radiusFull` is a `BorderRadius`, `UiConstants.radiusMdValue` is a `double`
+
+## Barrel Files (Staff Features)
+- Shifts: `lib/staff_shifts.dart` exports modules only
+- Payments: `lib/staff_payements.dart` (note: typo in filename) exports module only
+- Home: `lib/staff_home.dart` exports module only
+- These barrel files only export modules, not individual widgets -- skeleton widgets don't need to be added
+
+## Client App Feature Locations
+- Coverage: `packages/features/client/client_coverage/`
+- Home: `packages/features/client/home/` (no loading spinner -- renders default data during load)
+- Billing: `packages/features/client/billing/` (billing_page, pending_invoices_page, invoice_ready_page)
+- Reports: `packages/features/client/reports/` (reports_page with metrics_grid, plus 6 sub-report pages)
+- Reports barrel: `widgets/reports_page/index.dart`
+- Hubs: `packages/features/client/hubs/` (client_hubs_page + hub_details_page + edit_hub_page)
+
+## Key Patterns Observed
+- BenefitsOverviewPage also has CircularProgressIndicator (not shimmer-ified yet)
+- ShiftDetailsPage has a dialog-level spinner in the "applying" dialog -- this is intentional, not a page loading state
+- Hub details/edit pages use CircularProgressIndicator as action overlays (save/delete) -- keep as-is, not initial load
+- Client home page has no loading spinner; it renders with default empty dashboard data