Markdown in user messages, with a preview in the composer

User messages are rendered with the same Markdown component as the model's
answers; single line breaks are kept (hard breaks outside code blocks), so
plain multi-line messages look as before. The eye button next to the text
field shows the formatted message before sending. The text sent to the model
is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/i18n/en.ts

@@ -662,6 +662,9 @@ export const en = {
   "composer.mode": "Mode",
   "composer.model": "Model",
   "composer.permissionMode": "Permission mode",
+  "composer.preview": "Preview (Markdown)",
+  "composer.previewEdit": "Back to editing",
+  "composer.previewEmpty": "Nothing to preview yet.",
   "composer.attach": "Attach files",
 
   "command.compact": "Summarize the conversation to free up context",