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/styles.css

@@ -527,6 +527,20 @@ button.session-group:hover {
   word-break: break-word;
   position: relative;
 }
+/* User messages are Markdown too; the line breaks come from keepLineBreaks. */
+.msg-user .markdown {
+  white-space: normal;
+}
+.composer-preview {
+  min-height: 44px;
+  max-height: 40vh;
+  overflow-y: auto;
+  padding: 10px 12px;
+  border-radius: var(--radius);
+  border: 1px dashed var(--bg3);
+  background: var(--bg1);
+  cursor: text;
+}
 .msg-user .msg-actions {
   position: absolute;
   top: 4px;