Support pasted and dropped chat attachments

[ignaciojimenezr]

Summary

• add paste support for clipboard image/file attachments in the chat composer
• add drag/drop file attachment handling with an active drop overlay
• reuse existing attachment validation, previews, and submit conversion

Tests

• npm run test -- client/src/components/chat-v2/tests/ChatInput.test.tsx
• npm run typecheck:client

[chelojimenez]

✅ Snyk checks have passed. No issues have been found so far.

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✅	Open Source Security	0	0	0	0	0 issues

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[github-actions]

Internal preview

Preview URL: https://mcp-inspector-pr-2307.up.railway.app
Deployed commit: dda3462
PR head commit: f0a04e9
Backend target: staging fallback.
Health: ❌ Convex unreachable — see upsert-preview job logs (staging may need convex deploy)
Access is employee-only in non-production environments.