fix: add 3-way horizontal split variant and fix file manager overflow in split view (#796)

Adds a "3-Way (H)" layout option (2 top + 1 bottom) alongside the
existing "3-Way (V)" (1 left + 2 right).

Fixes file manager content overflowing its split pane by adding
min-h-0 to nested flex containers that were missing it.

src/ui/shell/SplitView.tsx

@@ -19,6 +19,8 @@ function defaultSizes(mode: SplitMode): {
       return { rowSizes: [100], rowColSizes: [[50, 50]] };
     case "3-way":
       return { rowSizes: [50, 50], rowColSizes: [[50, 50], [100]] };
+    case "3-way-horizontal":
+      return { rowSizes: [50, 50], rowColSizes: [[50, 50], [100]] };
     case "4-way":
       return {
         rowSizes: [50, 50],
@@ -444,6 +446,36 @@ export function SplitView({
         </div>
       )}
 
+      {splitMode === "3-way-horizontal" && (
+        <div className="flex flex-col w-full h-full min-h-0">
+          <div
+            className="flex min-h-0 overflow-hidden"
+            style={{ height: `${rowSizes[0]}%` }}
+          >
+            <div
+              className="min-w-0 min-h-0 overflow-hidden"
+              style={{ width: `${rowColSizes[0][0]}%` }}
+            >
+              {pane(0)}
+            </div>
+            <ColDivider
+              onMouseDown={(e) => onColDivider(e, 0, 0)}
+              onTouchStart={(e) => onColDividerTouch(e, 0, 0)}
+            />
+            <div className="flex-1 min-w-0 min-h-0 overflow-hidden">
+              {pane(1)}
+            </div>
+          </div>
+          <RowDivider
+            onMouseDown={(e) => onRowDivider(e, 0)}
+            onTouchStart={(e) => onRowDividerTouch(e, 0)}
+          />
+          <div className="flex-1 min-h-0 overflow-hidden">
+            {pane(2)}
+          </div>
+        </div>
+      )}
+
       {splitMode === "4-way" && (
         <div className="flex flex-col w-full h-full min-h-0">
           <Row rowIdx={0} paneIndices={[0, 1]} />