perf(buy-modal): derive oracle key with useMemo instead of effect + state (#543) - #632

Summary

BuyPolicyModal copied the oracle key into state from a useEffect:

useEffect(() => {
  if (product.category === 'crop') setOracleKey(buildRainfallKey(...));
  else if (product.category === 'flight') setOracleKey(buildFlightKey(...));
  else if (product.category === 'defi') setOracleKey('defi');
}, [product.category, lat, lng, year, month, flightNumber, flightDate]);

This caused two problems:

• Every keystroke in the latitude, longitude, year, month, flight number or date fields rendered the whole modal twice: once for the input change and again when the effect called setOracleKey.
• The first of those renders showed the previous key.

Fix

This follows the approach suggested in the issue:

• oracleKey is now computed during render with useMemo, from product.category and the builder inputs, so there's no effect and no extra render.
• Categories without a key builder (disaster, health, …) still need the free-text key input. That input now uses a separate state, manualOracleKey, and useMemo returns it for those categories.
• Everything downstream is unchanged: validation (32-char limit, required key), the "Computed Key" preview, the confirm step and invokeBuyPolicy / buyPolicy all still read oracleKey.

Tests

Added in src/__tests__/BuyPolicyModal.test.tsx:

• Typing a crop latitude updates the computed key right away (rainfall:1.5…)
• Typing a flight number updates the computed key (flight:KQ100:…)
• Manual key entry still works for a health product

npx vitest run src/__tests__/BuyPolicyModal.test.tsx
 Tests  7 failed | 6 passed (13)   # all 3 new tests pass

The 7 failures already happen on main, before this change. All of them are the existing renderToStaticMarkup tests. Modal renders nothing on the server, so the markup comes back as ''. purchase-flow.integration.test.tsx > shows validation error for empty coverage also fails on main. With this PR the set of failing tests is exactly the same as on main. Those tests are out of scope here and should be fixed separately.

tsc --noEmit reports no new errors in BuyPolicyModal.tsx. The existing makeProduct type error in the test file was already there.
Closes #543
Closes #540
Closes #541
Closes #542