fix(ui): clarify income tooltip & make mobile account layout responsive

[halfguru]

This PR implements minor mobile layout enhancements and tooltip clarifications for better UX:

1. Clarified Income Tooltip:

   • Updated the tooltip on "Annual Income" inside PersonForm.tsx to read: "Your current pre-tax annual income (used to calculate your retirement income target)" (resolving ambiguity regarding final salary prior to retirement).

2. Responsive Account Cards:

   • Refactored AccountCard.tsx layout to use a responsive flex grid (flex-col sm:flex-row).
   • Stacks the balance and contribution input fields vertically on mobile screens (<640px) so they expand to fill the full width of the card, eliminating the squished layout where inputs were compressed to ~35px wide.
   • Preserves the horizontal row layout for desktop screens.