- ChatLine gains sender_fp field for tracking who sent each message
- App gains read_receipts_sent HashSet to avoid duplicate receipts
- After each draw(), visible received messages get a Read receipt sent
- Only fires once per message_id, skips system/self messages
- Sender sees blue ✓✓ (existing display logic already handles Read)
- All ChatLine literals across 6 files updated with sender_fp field
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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