Improve mobile layout for exercise blocks (#35)

- stack exercise block headers on small screens to prevent inputs and controls from overlapping
- allow exercise controls to take full width and align left for better mobile spacing

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[Codex Task](https://chatgpt.com/codex/tasks/task_e_6922e531f9988325b7338b2a7df5a3c5)

Author: rodrigogiraoserrao

workout-manager.html

@@ -159,6 +159,22 @@
                 grid-template-columns: 1.1fr 1fr;
             }
         }
+
+        @media (max-width: 720px) {
+            .exercise-block header {
+                flex-direction: column;
+                align-items: stretch;
+            }
+
+            .exercise-name-row,
+            .exercise-controls {
+                width: 100%;
+            }
+
+            .exercise-controls {
+                justify-content: flex-start;
+            }
+        }
     </style>
 </head>