[Talentgift24-ux]

#1013 fix(gift): every well-formed gift code is rejected as "not recognized" because of a length check that can never pass

Close#1013
RedeemGift validated codes against ^GIFT-[A-Z0-9]{6}-[A-Z0-9]{6}$ (always 18
characters) and then rejected anything shorter than 20 characters. Every code that
passed the format check therefore failed the length check, so the redemption flow was
unreachable — including codes handed out by GiftPurchaseModal. The resolved payload was
also discarded and replaced with a duplicated hardcoded gift object.

Changes

• New src/utils/giftCode.ts — owns code normalisation, generation and validation:
  • normalizeGiftCode() uppercases, strips whitespace/separators, and accepts the code
    with or without dashes and with or without the GIFT prefix, so
    gift-abc123-xyz789, GIFT ABC123 XYZ789 and ABC123XYZ789 all resolve to
    GIFT-ABC123-XYZ789. Replaces the old formatCode(), which blindly prepended
    GIFT- and produced GIFT-GIFTABC123XYZ789 for pasted codes.
  • generateGiftCode() — crypto-random, always emits a canonical 12-character body. The
    previous Math.random().toString(36).substring(2, 8) generator could emit short
    segments and codes that did not match the format.
  • validateGiftCode() — returns a typed result (ok | invalid | expired | redeemed)
    with the resolved gift details, instead of rejecting with a thrown object.
• RedeemGift.tsx — renders the gift details that validation actually resolved (the
  duplicated mockGiftDetails object is gone), canonicalises the input on blur, and guards
  the ?code= auto-validation with a ref so a StrictMode double mount starts exactly one
  validation.
• GiftPurchaseModal.tsx — uses the shared generateGiftCode(), so a purchased code is
  guaranteed to be redeemable.
• RedeemConfirmModal.tsx — imports the single shared GiftDetails type instead of a
  second copy.

Testing

• New src/utils/giftCode.test.ts: normalisation variants, malformed input, generated-code
  format/uniqueness, and each validateGiftCode outcome (ok / expired / redeemed /
  not_found / malformed).

• src/pages/RedeemGift.test.tsx: added a regression test that a code produced by
  GiftPurchaseModal opens RedeemConfirmModal, the /redeem-gift?code=GIFT-ABC123-XYZ789
  repro, pasted-variant canonicalisation, and a StrictMode single-validation check. The old
  formats code input automatically case asserted the buggy GIFT-ABC123XYZ789 output and
  now asserts canonicalisation on blur.

• I have not been able to run vitest locally (dependency install kept failing on a
  flaky network), so the new tests are unverified here — please treat the CI run as the
  first real signal.

Out of scope

• A real gift-validation endpoint (validateGiftCode remains a placeholder service with the
  same latency and fixture codes as before).
• GiftRedeemSuccess rendering.