fix: scroll wallet screens behind top bar with haze blur

[jvsena42]

This PR:

1. Makes the Spending and Savings wallet screen content scroll behind the top bar, matching the iOS scroll behavior
2. Applies a haze blur effect with a gradient mask to the AppTopBar on both screens
3. Extracts the top bar gradient as a shared constant reused by HomeScreen, SavingsWalletScreen, and SpendingWalletScreen

Description

The wallet screens previously used a fixed ScreenColumn layout where the AppTopBar sat above a non-scrollable Column. Content could not scroll behind the top bar, and the top bar had no blur effect.

This replaces the ScreenColumn approach with a Box-based layout: the AppTopBar floats as an overlay with hazeEffect + gradient mask + zIndex(1f), and all content (balance, transfer buttons, activity list) is unified into a single LazyColumn that scrolls underneath. A new LazyListScope.activityListGroupedItems() extension inlines activity items directly into the parent LazyColumn, avoiding nested scrollable containers.

Preview

QA Notes

1. Open Savings wallet screen with a balance and activity
   • Scroll down and verify the activity list and balance header scroll behind the top bar
   • Verify the top bar shows a frosted glass blur effect with a gradient fade
   • Verify the back button and drawer icon remain tappable

savings.webm

2. Open Spending wallet screen with a balance and activity
   • Same scroll-behind and blur behavior as Savings
   • Verify "Transfer To Savings" button scrolls with content

spending.webm

3. Open Savings/Spending wallet with no balance and no activity
   • Verify the empty state onboarding view still displays correctly at the bottom

empty-states.webm

4. Verify HomeScreen top bar blur still works as before (shared gradient constant)

home.webm

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.