Add safety number verification UI

[deep-bhikadiya]

Summary

• render contact safety numbers in the conversation screen
• persist local verified state per contact fingerprint
• warn and prompt re-verification when a contact key set changes

Testing

• npm run lint (apps/web)
• ./node_modules/.bin/tsc --noEmit (apps/web)

Closes #301

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[deep-bhikadiya]

@codebestia please check this.