feat: End-to-End Message Encryption (E2EE) via Web Crypto API

[knoxiboy]

Closes #567

Changes

• Implement E2EE using the Web Crypto API (AES-GCM + ECDH key exchange)
• Encrypt messages before sending, decrypt on receive
• Store keys securely in memory (never sent to server)

Type of Change

• New feature (security enhancement)

Fixes #567

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: e277468b-fe98-46d2-a4a8-80229a589a49

📥 Commits

Reviewing files that changed from the base of the PR and between b6ebd50 and 4a7ced5.

📒 Files selected for processing (9)

• backend/src/controllers/auth.controller.js
• backend/src/controllers/message.controller.js
• backend/src/models/message.model.js
• backend/src/models/user.model.js
• backend/src/routes/auth.route.js
• frontend/components/chat/ChatWindow.jsx
• frontend/lib/crypto.js
• frontend/src/App.jsx
• frontend/src/store/useChatStore.js

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