fix: timezone-safe dates, next/font loading, accessible accordions,

[Maxwell316]

Summary

This PR fixes four issues: one date bug, one performance problem, one accessibility gap and one input validation bug.

🐛 DatePicker timezone handling (#778)

• Store and compare dates as UTC. Date input values (YYYY-MM-DD) are parsed as UTC midnight instead of local time, so a policy's start and end dates no longer shift by a day for users ahead of UTC.
• Show dates in the user's local timezone everywhere, using the shared helpers in src/lib/format.ts.
• Add parse/serialize helpers for date inputs and use them in BuyPolicyModal.
• Document how timezones are handled (in format.ts and the README).
• Tests: run with a positive UTC offset (UTC+13, Pacific/Auckland) and check that the date the user picks is the date that gets stored.

⚡ Font loading blocks first paint (#780)

• Move fonts to next/font in app/layout.tsx and remove the stylesheet/CDN font loading that blocks rendering.
• Set display: 'swap' and preload the critical font weights.
• Expose the font through a CSS variable so globals.css and Tailwind pick it up.
• Lighthouse before and after: see the table below.

♿ Accordion keyboard and ARIA support (#781)

• Make every accordion toggle a <button type="button"> (some are currently divs with only onClick).
• Add aria-expanded and aria-controls, with a matching id on the content region.
• Enter and Space now open and close sections, and the toggles show a visible focus style.
• Tests: check that the toggle can be reached with the keyboard and that aria-expanded changes.

🐛 NumberInput accepts scientific notation (#782)

• Amount inputs (BuyPolicyModal, DepositModal, WithdrawModal) now reject e, E, + and - as the user types and when text is pasted.
• Check the number format again on submit with a strict decimal pattern.
• Show a clear validation error when the format is invalid.
• Tests: check that 1e10 is rejected and the error message appears.

Lighthouse (#780)

Metric	Before	After
FCP
LCP
CLS

Test plan

• yarn test passes
• yarn lint passes
• Set the system timezone to UTC+13, pick a policy start date, and confirm the stored date has not shifted
• Tab to the expandable sections and open and close them with Enter and Space
• Type or paste 1e10 into an amount field and confirm it is rejected with an error
• Run Lighthouse before and after and fill in the table above

Closes #778
Closes #780
Closes #781
Closes #782

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