fix: nested split pane flex squeezing and add divider drag resize

src/renderer/css/split.css

@@ -14,8 +14,11 @@
 
 .split-container {
   display: flex;
+  flex: 1 1 0%;
   width: 100%;
   height: 100%;
+  min-width: 0;
+  min-height: 0;
   overflow: hidden;
   position: relative;
 }
@@ -29,14 +32,14 @@
 }
 
 .split-pane {
-  flex: 1;
+  flex: 1 1 0%;
   display: flex;
   position: relative;
   overflow: hidden;
   border: 1px solid transparent;
   transition: border-color 0.15s ease, box-shadow 0.15s ease;
-  min-width: 120px;
-  min-height: 80px;
+  min-width: 0;
+  min-height: 0;
 }
 
 .split-pane.focused {
@@ -49,6 +52,7 @@
   height: 100%;
   padding: 6px 10px;
   overflow: hidden;
+  box-sizing: border-box;
 }
 
 .xterm {
@@ -65,19 +69,24 @@
 .split-divider {
   background: var(--border-color);
   z-index: 10;
-  transition: background 0.2s ease;
+  flex-shrink: 0;
+  flex-grow: 0;
+  transition: background 0.15s ease, box-shadow 0.15s ease;
+  position: relative;
 }
 
 .split-container.horizontal > .split-divider {
-  width: 2px;
+  width: 4px;
   cursor: col-resize;
 }
 
 .split-container.vertical > .split-divider {
-  height: 2px;
+  height: 4px;
   cursor: row-resize;
 }
 
-.split-divider:hover {
+.split-divider:hover,
+.split-divider.dragging {
   background: var(--accent-color);
+  box-shadow: 0 0 10px var(--accent-glow);
 }