Verify encryption keys (safety numbers / QR)

[mrsumanbiswas]

Allow users to verify the identity of their contacts and ensure no man‑in‑the‑middle.

• Show a unique “safety number” (or QR code) for each contact in the conversation settings
• Let users scan a contact’s QR code to verify in person
• Mark a contact as “verified” after successful verification
• Display a verified badge/icon next to the contact’s name

Implement the comparison flow clearly and make it easy to perform during an in‑person meeting.

[mrsumanbiswas]

backend part done with #116