feat: add real-time "seen" status for messages

[jessmed749]

Feature: Seen Status for Messages

Overview

Adds real-time "seen" indicators to messages, similar to iMessage/WhatsApp behavior.

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Changes

page.tsx

• Fixed subscription callback to properly handle UPDATE events
• Ensures is_read updates propagate in real-time via Supabase
• When a receiver opens a chat, markMessagesAsRead triggers an UPDATE that syncs across clients

ChatWindow.tsx

• Added "Seen" label below messages
• Displays only on the most recent outgoing message where is_read === true
• Mimics modern messaging UX (only last seen message shows status)

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Behavior

• When receiver opens chat → messages marked as read
• Supabase UPDATE triggers → sender UI updates instantly
• "Seen" appears under latest read message only

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Notes

• Uses existing is_read field
• No breaking changes