feat: add @mention parsing, chip rendering, and notification integration

[Keengfk]

Add @mention parsing, chip rendering & notification integration to
CommentSection

Closes #

What changed

• notifications.ts — added parseMentions(text) (extracts unique valid Stellar
  addresses via /\bG[A-Z0-9]{55}\b/g) and notifyMention(mentionedAddress,
  commenterAddress, invoiceId) (fires a browser notification, skips
  self-mentions)
• CommentSection.tsx — renderCommentText() splits comment text on the mention
  pattern and renders valid @g... addresses as truncated indigo chips with
  full-address aria-label; handleSubmit calls parseMentions + notifyMention for
  each mentioned address on submit
• CommentSection.test.tsx — 13 unit tests covering mention parsing (no match,
  single, multi, dedup, malformed short/long/lowercase), chip rendering (plain
  text passthrough, single chip, multi chip, malformed stays plain), and
  notification dispatch (self-suppression, fires for others)

Acceptance criteria met

• ✅ @g... patterns render as distinct styled inline chips
• ✅ Submitting triggers a notification per mentioned address via existing
  notification plumbing
• ✅ Self-mentions do not trigger a notification
• ✅ Malformed @ patterns render as plain text
• ✅ Unit tests pass (13/13)
  closes Build comment thread notifications with @mentions #256

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