fix(FN-1123): correct branches panel flex sizing

- Make .gm-content a column flex container so nested panels can size correctly
- Let .gm-panel grow/shrink with flex: 1 and min-height: 0 to avoid layout clipping
- Replace fixed .gm-branches-list max-height with flex-based scrolling to use available height

packages/dashboard/app/styles.css

@@ -15674,6 +15674,8 @@ html .column.drag-over * {
 
 .gm-content {
   flex: 1;
+  display: flex;
+  flex-direction: column;
   overflow-y: auto;
   padding: var(--space-lg);
   min-height: 400px;
@@ -15712,7 +15714,9 @@ html .column.drag-over * {
 
 .gm-panel {
   display: flex;
+  flex: 1;
   flex-direction: column;
+  min-height: 0;
   gap: var(--space-md);
 }
 
@@ -16392,8 +16396,9 @@ html .column.drag-over * {
 
 .gm-branches-list {
   display: flex;
+  flex: 1;
   flex-direction: column;
-  max-height: 350px;
+  min-height: 0;
   overflow-y: auto;
 }