refactor(frontend): simplify event chat UI

- Replace "Edit heats" button with icon-only version
- Remove connection status indicator (show "Disconnected" warning only when offline)
- Remove event location from header
- Hide Participants tab on desktop (sidebar already visible)
- Remove "Participants" header from sidebar on desktop

frontend/src/components/events/ParticipantsSidebar.jsx

@@ -37,7 +37,8 @@ const ParticipantsSidebar = ({
   onHideMyHeatsChange,
   onMatchWith,
   className = '',
-  fullWidth = false
+  fullWidth = false,
+  showHeader = true
 }) => {
   const participantCount = users.length;
   const onlineCount = activeUsers.length;
@@ -45,16 +46,20 @@ const ParticipantsSidebar = ({
   return (
     <div className={`${fullWidth ? 'w-full' : 'w-64 border-r'} bg-gray-50 p-4 overflow-y-auto ${className}`}>
       {/* Header */}
-      <div className="mb-4">
-        <h3 className="font-semibold text-gray-900 mb-2">
-          Participants ({participantCount})
-        </h3>
-        <p className="text-xs text-gray-500 mb-3">
-          {onlineCount} online
-        </p>
+      {showHeader && (
+        <div className="mb-4">
+          <h3 className="font-semibold text-gray-900 mb-2">
+            Participants ({participantCount})
+          </h3>
+          <p className="text-xs text-gray-500 mb-3">
+            {onlineCount} online
+          </p>
+        </div>
+      )}
 
-        {/* Filter Checkbox */}
-        {myHeats.length > 0 && (
+      {/* Filter Checkbox */}
+      {myHeats.length > 0 && (
+        <div className="mb-4">
           <label className="flex items-center gap-2 text-sm text-gray-700 cursor-pointer hover:text-gray-900">
             <input
               type="checkbox"
@@ -65,8 +70,8 @@ const ParticipantsSidebar = ({
             <Filter className="w-3 h-3" />
             <span>Hide users from my heats</span>
           </label>
-        )}
-      </div>
+        </div>
+      )}
 
       {/* Empty State */}
       {participantCount === 0 && (