feat: add activity list skeleton loading state

[ghg001]

Overview

This PR adds a skeleton loading state for the activity list that matches the app's visual style. While transaction history is loading, users see neutral placeholder rows instead of a blank area or generic spinner. The skeleton is replaced by real transactions when data arrives, and by the empty state when there are no transactions.

Related Issue

Closes #

Changes

🏗️ Activity Skeleton Loading State

• [ADD] src/components/ActivitySkeleton.tsx

  • Neutral placeholder rows that mirror the activity list item layout.
  • Uses the app's existing design tokens and colors for a consistent look.
  • Light shimmer animation communicates loading without a generic spinner.
  • Contains only grey placeholder blocks — no fake transaction text or values.

• [MODIFY] app/(tabs)/history.tsx

  • Renders <ActivitySkeleton /> while the transaction history query is loading.
  • Replaces the skeleton with the transaction list once data loads.
  • Replaces the skeleton with the empty state when there are no transactions.

• [MODIFY] src/components/index.ts

  • Exports ActivitySkeleton from the shared components barrel for reuse.

Verification Results

npm run lint
✅ All checks pass

Manual acceptance check:
✅ Skeleton visible while activity data is loading
✅ Skeleton matches app style (neutral placeholder content)
✅ Skeleton replaced by transactions after load
✅ Skeleton replaced by empty state when no transactions
✅ No fake transaction data shown

Acceptance Criteria	Status
Activity loading state is visible	✅ Skeleton renders immediately during fetch
Loading state matches the app style	✅ Uses existing design tokens and neutral grey blocks
Loading state is replaced by transactions when data loads	✅ Transaction list renders after fetch resolves
Loading state is replaced by empty state when there are no transactions	✅ Empty state renders when transaction list is empty
Loading state does not show fake real transaction data	✅ Placeholder rows contain only neutral blocks

Closes #99