Build comment thread notifications with @mentions

[Kingsman-99]

Description

CommentSection.tsx supports adding comments but has no @mention or notification-on-mention capability — a mentioned recipient has no way to know they were tagged. This issue adds @mention parsing and notification integration.

Technical Context

Involves src/components/CommentSection.tsx and src/lib/notifications.ts. Parse @G... (Stellar address) mentions in comment text, rendering them as styled chips; on submit, trigger a notification (via existing notifications.ts mechanism) to each mentioned address.

Acceptance Criteria

• @G... patterns in comment text render as distinct styled inline chips, not plain text
• Submitting a comment with mentions triggers a notification per mentioned address via existing notification plumbing
• Self-mentions (commenter mentioning their own address) do not trigger a self-notification
• Malformed @ patterns (not valid Stellar addresses) render as plain text, not broken chips
• Unit tests: mention parsing/chip rendering, self-mention suppression, malformed pattern handling

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