feat: validate CreateVault form inputs

[go165]

Closes #123.

Summary

• Add pure CreateVault validation helpers for USDC amount precision, future deadlines, Stellar public key format, and distinct success/failure destinations.
• Wire validation into CreateVault so invalid values render inline Field errors and block submission before backend/contract calls.
• Document the Field validation/error pattern.
• Fix the existing Field forwardRef syntax so the validation path can compile and be tested.

Validation

• npm test -- --run src/utils/__tests__/vaultValidation.test.ts src/pages/__tests__/CreateVault.test.tsx

Focused result: 2 files passed, 7 tests passed; CreateVault.tsx and vaultValidation.ts both reported 100% coverage in the focused run.

Baseline notes

• npm run build is currently blocked by pre-existing repo-wide TypeScript issues outside this PR, including missing Vitest globals in existing tests, an existing @testing-library/user-event import without a dependency in Vaults.test.tsx, and unrelated unused/missing imports.
• npm run lint is likewise blocked by existing repo-wide lint errors outside these changed TypeScript files. A scoped lint on the changed TS files passed; the markdown doc is ignored by the current ESLint config.

Reward/payment note if this issue is selected for GrantFox reward: Base/EVM address 0x1f0130669ca6fd02e025a984cc038f139df19a2f.