feat(chat): add secure dm frontend foundation

[creativaPoeta]

Summary

Add the frontend foundation for secure_dm_v1.

Included

• secure device bootstrap on authenticated clients
• local WebCrypto-based secure message encrypt/decrypt flow for text-only DMs
• secure chat fetch/send/read client services
• secure socket awareness for secure_message_available
• explicit UI entry points for secure chat
• preferred secure-first DM opening with legacy fallback from key entry points

Notes

• this is a foundation PR, not the final secure chat rollout
• legacy chat remains intact
• secure DMs still depend on the matching backend foundation branch
• build validated locally with npm run build

Follow-up

• integrate and deploy matching backend secure branch
• run end-to-end secure DM testing on deployed environments
• decide rollout UX and migration path for existing legacy DMs

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