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/flat-page-layout-widget/services/flat-page-layout-widget-type-validator.service.ts

@@ -92,6 +92,9 @@ export class FlatPageLayoutWidgetTypeValidatorService {
       RECORD_TABLE: validateSimpleRecordPageWidgetForCreation(
         WidgetConfigurationType.RECORD_TABLE,
       ),
+      EMAIL_THREAD: validateSimpleRecordPageWidgetForCreation(
+        WidgetConfigurationType.EMAIL_THREAD,
+      ),
     };
 
   private readonly PAGE_LAYOUT_WIDGET_TYPE_VALIDATOR_FOR_UPDATE_HASHMAP: FlatPageLayoutWidgetTypeValidatorForUpdate =
@@ -143,6 +146,9 @@ export class FlatPageLayoutWidgetTypeValidatorService {
       RECORD_TABLE: validateSimpleRecordPageWidgetForUpdate(
         WidgetConfigurationType.RECORD_TABLE,
       ),
+      EMAIL_THREAD: validateSimpleRecordPageWidgetForUpdate(
+        WidgetConfigurationType.EMAIL_THREAD,
+      ),
     };
 
   public validateFlatPageLayoutWidgetTypeSpecificitiesForCreation(

packages/twenty-server/src/engine/metadata-modules/flat-page-layout-widget/utils/from-page-layout-widget-configuration-to-universal-configuration.util.ts

@@ -378,6 +378,7 @@ export const fromPageLayoutWidgetConfigurationToUniversalConfiguration = ({
     case WidgetConfigurationType.WORKFLOW_RUN:
     case WidgetConfigurationType.IFRAME:
     case WidgetConfigurationType.STANDALONE_RICH_TEXT:
+    case WidgetConfigurationType.EMAIL_THREAD:
       return configuration;
   }
 };