feat(ui): submit task composers with Enter by default (#19272)

Use Enter to submit task replies and new tasks, with Shift+Enter for a new line. Add an experimental preference to require Command/Ctrl+Enter.

Preserve autocomplete selection and IME confirmation, and retain the modifier shortcut until saved settings load.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/shared/src/validators/instance.ts

@@ -69,6 +69,7 @@ export const instanceExperimentalSettingsSchema = z.object({
   enableArtifactsV1: z.boolean().default(true),
   enableConferenceRoomChat: z.boolean().default(false),
   enableClassicTaskInterface: z.boolean().default(false),
+  enableLegacyComposerSubmit: z.boolean().default(false),
   enableIssuePlanDecompositions: z.boolean().default(false),
   enableExperimentalFileViewer: z.boolean().default(false),
   enableExternalObjects: z.boolean().default(false),