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-front/src/modules/page-layout/utils/getDefaultRecordPageLayoutId.ts

@@ -1,5 +1,6 @@
 import { CoreObjectNameSingular } from 'twenty-shared/types';
 import { DEFAULT_COMPANY_RECORD_PAGE_LAYOUT_ID } from '@/page-layout/constants/DefaultCompanyRecordPageLayoutId';
+import { DEFAULT_MESSAGE_THREAD_RECORD_PAGE_LAYOUT_ID } from '@/page-layout/constants/DefaultMessageThreadRecordPageLayoutId';
 import { DEFAULT_NOTE_RECORD_PAGE_LAYOUT_ID } from '@/page-layout/constants/DefaultNoteRecordPageLayoutId';
 import { DEFAULT_OPPORTUNITY_RECORD_PAGE_LAYOUT_ID } from '@/page-layout/constants/DefaultOpportunityRecordPageLayoutId';
 import { DEFAULT_PERSON_RECORD_PAGE_LAYOUT_ID } from '@/page-layout/constants/DefaultPersonRecordPageLayoutId';
@@ -20,6 +21,8 @@ const OBJECT_NAME_TO_DEFAULT_LAYOUT_ID: Record<string, string> = {
   [CoreObjectNameSingular.WorkflowVersion]:
     DEFAULT_WORKFLOW_VERSION_PAGE_LAYOUT_ID,
   [CoreObjectNameSingular.WorkflowRun]: DEFAULT_WORKFLOW_RUN_PAGE_LAYOUT_ID,
+  [CoreObjectNameSingular.MessageThread]:
+    DEFAULT_MESSAGE_THREAD_RECORD_PAGE_LAYOUT_ID,
 };
 
 export const getDefaultRecordPageLayoutId = ({