Fix chat send/stop buttons squashing on mobile

Add min-width to circular buttons in ChatView and ChatPanel to prevent
flex compression on narrow viewports.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/ChatPanel.vue

@@ -598,6 +598,7 @@ function excludeAutoNote(noteId: number) {
 }
 .btn-send {
   width: 30px;
+  min-width: 30px;
   height: 30px;
   padding: 0;
   display: flex;

frontend/src/views/ChatView.vue

@@ -865,6 +865,7 @@ function excludeAutoNote(noteId: number) {
 }
 .btn-send {
   width: 34px;
+  min-width: 34px;
   height: 34px;
   padding: 0;
   display: flex;
@@ -884,6 +885,7 @@ function excludeAutoNote(noteId: number) {
 }
 .btn-stop {
   width: 34px;
+  min-width: 34px;
   height: 34px;
   padding: 0;
   display: flex;