Email validation, rate-limit feedback, rounding handler, and rich text editor

[Almikefred]

Summary

This PR implements four critical features for invoice management: email validation with MX checking, rate-limit feedback with countdown timer, split amount rounding discrepancy resolution, and rich text notes editor with sanitization.

Changes

#498 — Recipient Email and Contact Validation

• Added src/app/api/validate-email/route.ts for DNS MX record checking
• Created useEmailValidation hook with 500ms debounce and 5-minute cache per domain
• Added EmailField component with real-time validation feedback
• Email validation errors show on blur, warnings for missing MX records, green checkmark for valid emails

#499 — Invoice Creation Rate-Limit Feedback UI

• Created useInvoiceForm hook to handle 429 responses and countdown state management
• Added RateLimitBanner component showing live countdown timer (updates every second)
• Displays retry-after time extracted from headers or shows static message if absent

#500 — Split Amount Rounding Discrepancy Handler and Warning

• Added resolveRounding function to useSplitCalculator that handles stroop rounding and redistributes discrepancies
• Created SplitSummaryCard component showing warning when rounding adjustment applied
• Added validateRecipientAmountSum to src/lib/stellar.ts for transaction validation
• Comprehensive unit tests covering 3, 7, and 11 recipient splits

#501 — Rich Text Notes Editor for Invoice Description

• Created NotesEditor component using Tiptap with StarterKit extension
• Toolbar with bold, italic, bullet list, and link buttons (keyboard navigable)
• Added htmlSanitizer utility for DOMPurify sanitization on input and output
• Safe HTML rendering on public invoice pages

Test Coverage

• Unit tests for rounding resolution across various recipient counts and percentage distributions

Notes

• Email validation requires @tiptap/react, @tiptap/starter-kit, @tiptap/extension-link, and dompurify packages (to be installed separately)
• All HTML output is sanitized with DOMPurify using allowlist for specific tags and attributes
• Rate-limit countdown updates every second with proper cleanup of intervals

Closes #498, Closes #499, Closes #500, Closes #501

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