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/workspace-manager/workspace-migration/universal-flat-entity/constants/__tests__/__snapshots__/all-universal-flat-entity-foreign-key-aggregator-properties.constant.spec.ts.snap

@@ -1,4 +1,4 @@
-// Jest Snapshot v1, https://goo.gl/fbAQLP
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
 
 exports[`ALL_UNIVERSAL_FLAT_ENTITY_FOREIGN_KEY_AGGREGATOR_PROPERTIES should match snapshot 1`] = `
 {

packages/twenty-server/src/engine/workspace-manager/workspace-migration/workspace-migration-runner/action-handlers/page-layout-widget/services/utils/from-universal-configuration-to-flat-page-layout-widget-configuration.util.ts

@@ -361,6 +361,7 @@ export const fromUniversalConfigurationToFlatPageLayoutWidgetConfiguration = ({
     case WidgetConfigurationType.WORKFLOW_RUN:
     case WidgetConfigurationType.IFRAME:
     case WidgetConfigurationType.STANDALONE_RICH_TEXT:
+    case WidgetConfigurationType.EMAIL_THREAD:
       return universalConfiguration;
   }
 };