feat(project-chat): add task detail chat with @mentions and full backend integration

[ganeshwhere]

Summary

This PR introduces project chat as part of the task detail experience, with end-to-end backend integration and UI updates.

What’s Included

• Added backend project chat module in API:
  • GET /projects/:projectId/chat/messages
  • POST /projects/:projectId/chat/messages
• Added Prisma schema + migration for:
  • ProjectChatMessage
  • ProjectChatMention
• Added mention support (@user) with mention-user resolution and persistence.
• Integrated chat into task detail page (right-side chat pane).
• Removed chat from board page and kept chat focused on task detail.
• Upgraded task detail layout:
  • Better meta cards (Priority, Status, Due Date, Assignee)
  • Cleaner information hierarchy and spacing
• Improved chat composer UX:
  • WhatsApp-style bubble layout
  • Auto-resizing input
  • Better send button states/loading
  • Consistent icon sizing

Reliability/Validation

• Added/updated tests for chat actions and service behavior.
• Added safer error normalization on web action layer.
• Added Prisma runtime handling for missing chat-table migration cases.
• Verified:
  • pnpm lint
  • pnpm test
  • pnpm build

Migration Note

Run DB migration before testing chat in environments using this branch:

• pnpm --filter api prisma migrate deploy
• pnpm --filter api prisma generate

Impact

• No breaking API changes to existing endpoints.
• New DB objects are introduced for chat support.