Add email thread widget and message thread record page layout (#19351)

## Summary
- Move email thread display from side panel to a dedicated record page
with a new `EMAIL_THREAD` widget type
- Add message thread as a standard object with page layout, subject
field, and backfill command
- Add reply-to-email command menu item for message thread records
- Remove old side panel message thread components in favor of the new
widget-based approach

## Type fixes
- Add `EMAIL_THREAD` to `WidgetConfigurationType`, `WidgetType`, and all
configuration/validator maps
- Create `EmailThreadConfigurationDTO` and shared
`EmailThreadConfiguration` type
- Register EMAIL_THREAD in widget type validators, configuration
resolvers, and standard widget mappings

## Test plan
- [ ] Verify message thread record pages render with the email thread
widget
- [ ] Verify email thread preview navigates to the record page instead
of opening side panel
- [ ] Verify reply-to-email command appears for message thread records
- [ ] Verify typecheck passes for both twenty-front and twenty-server
- [ ] Run existing test suites to check for regressions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/twenty-server/src/engine/metadata-modules/command-menu-item/enums/engine-component-key.enum.ts

@@ -55,6 +55,7 @@ export enum EngineComponentKey {
   VIEW_PREVIOUS_AI_CHATS = 'VIEW_PREVIOUS_AI_CHATS',
   TRIGGER_WORKFLOW_VERSION = 'TRIGGER_WORKFLOW_VERSION',
   FRONT_COMPONENT_RENDERER = 'FRONT_COMPONENT_RENDERER',
+  REPLY_TO_EMAIL_THREAD = 'REPLY_TO_EMAIL_THREAD',
 
   // Deprecated keys kept for backward compatibility until migration runs
   DELETE_SINGLE_RECORD = 'DELETE_SINGLE_RECORD',