fix(app): distinguish delete tabs

[daniellam258]

Summary

• explicitly style inactive delete-memory tabs with a black background and white text
• keep the selected tab white with black text
• assert that the selected tab state flips correctly when switching tabs

Root cause

The first fix defined only the selected style. The inactive tab still inherited a white background from the broader button styling, so both tabs appeared selected.

Impact

Only the active tab is now visually highlighted, making the current delete-memory view clear.

Validation

• pnpm --filter @memwal/app test -- src/components/SecurityDeleteSection.test.tsx
• targeted ESLint check for SecurityDeleteSection.test.tsx
• git diff --check