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-client-sdk/src/metadata/generated/schema.graphql

@@ -921,6 +921,7 @@ enum WidgetType {
   WORKFLOW_RUN
   FRONT_COMPONENT
   RECORD_TABLE
+  EMAIL_THREAD
 }
 
 union PageLayoutWidgetPosition = PageLayoutWidgetGridPosition | PageLayoutWidgetVerticalListPosition | PageLayoutWidgetCanvasPosition
@@ -948,7 +949,7 @@ type PageLayoutWidgetCanvasPosition {
   layoutMode: PageLayoutTabLayoutMode!
 }
 
-union WidgetConfiguration = AggregateChartConfiguration | StandaloneRichTextConfiguration | PieChartConfiguration | LineChartConfiguration | IframeConfiguration | GaugeChartConfiguration | BarChartConfiguration | CalendarConfiguration | FrontComponentConfiguration | EmailsConfiguration | FieldConfiguration | FieldRichTextConfiguration | FieldsConfiguration | FilesConfiguration | NotesConfiguration | TasksConfiguration | TimelineConfiguration | ViewConfiguration | RecordTableConfiguration | WorkflowConfiguration | WorkflowRunConfiguration | WorkflowVersionConfiguration
+union WidgetConfiguration = AggregateChartConfiguration | StandaloneRichTextConfiguration | PieChartConfiguration | LineChartConfiguration | IframeConfiguration | GaugeChartConfiguration | BarChartConfiguration | CalendarConfiguration | FrontComponentConfiguration | EmailsConfiguration | EmailThreadConfiguration | FieldConfiguration | FieldRichTextConfiguration | FieldsConfiguration | FilesConfiguration | NotesConfiguration | TasksConfiguration | TimelineConfiguration | ViewConfiguration | RecordTableConfiguration | WorkflowConfiguration | WorkflowRunConfiguration | WorkflowVersionConfiguration
 
 type AggregateChartConfiguration {
   configurationType: WidgetConfigurationType!
@@ -989,6 +990,7 @@ enum WidgetConfigurationType {
   WORKFLOW_RUN
   FRONT_COMPONENT
   RECORD_TABLE
+  EMAIL_THREAD
 }
 
 type StandaloneRichTextConfiguration {
@@ -1154,6 +1156,10 @@ type EmailsConfiguration {
   configurationType: WidgetConfigurationType!
 }
 
+type EmailThreadConfiguration {
+  configurationType: WidgetConfigurationType!
+}
+
 type FieldConfiguration {
   configurationType: WidgetConfigurationType!
   fieldMetadataId: String!
@@ -2632,6 +2638,7 @@ enum EngineComponentKey {
   VIEW_PREVIOUS_AI_CHATS
   TRIGGER_WORKFLOW_VERSION
   FRONT_COMPONENT_RENDERER
+  REPLY_TO_EMAIL_THREAD
   DELETE_SINGLE_RECORD
   DELETE_MULTIPLE_RECORDS
   RESTORE_SINGLE_RECORD