Fix flag picker overlap with share actions

[ahfoysal]

What I changed

• Adjusted country picker position logic in components/FlagPicker.tsx.
• The picker now checks available space and opens upward when there is not enough room below the trigger.
• This prevents the dropdown from covering the share buttons (issue UI Issue: "Share My Card" button overlaps the country dropdown #35) while preserving the existing keyboard and ARIA behavior.

Validation

• npm run lint
• npm test