mobile: audit touch target sizes in bottom tabs

Goal

Guarantee that all five bottom-navigation tabs expose a stable touch target of at least 48 logical pixels high. The target should remain large enough even if icon, font, or label metrics change later.

Where to work

• apps/mobile/App.tsx around the TABS.map(...) bottom-navigation buttons
• apps/mobile/scripts/verify-frontend-regressions.mjs for a focused regression guard

Suggested scope

1. Give each bottom-tab TouchableOpacity an explicit minHeight: 48 (or an equivalent stable layout rule) while preserving the five equal-width tabs.
2. Keep the existing hitSlop, accessibilityRole, accessibilityLabel, and selected accessibilityState behavior.
3. Extend the frontend regression script so removing the minimum target-size guarantee fails npm run verify.

Acceptance criteria

• Each of the five bottom tabs has a stable target height of at least 48 logical pixels.
• Labels and icons remain centered and do not overlap at narrow mobile width.
• Existing accessibility metadata remains intact.
• cd apps/mobile && npm run verify passes.
• python3 scripts/check_public_tree.py passes.
• The PR body includes Closes #13 and briefly explains the accessibility benefit.

Non-goals

• Do not replace the navigation architecture or redesign the tab bar.
• Do not add screenshots, generated assets, local engines, or private files.

Comment /claim before starting so another contributor is not assigned the same first issue.