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/features/file-manager/FileManager.tsx

@@ -2551,7 +2551,7 @@ function FileManagerContent({ initialHost, onClose }: FileManagerProps) {
   return (
     <div className="h-full flex flex-col bg-background relative overflow-hidden isolate">
       <div
-        className="h-full w-full flex flex-col"
+        className="h-full w-full flex flex-col min-h-0"
         style={{
           visibility: isConnectionLogExpanded ? "hidden" : "visible",
         }}
@@ -2855,7 +2855,7 @@ function FileManagerContent({ initialHost, onClose }: FileManagerProps) {
                 : "hidden md:flex",
             )}
           >
-            <div className="flex-1 flex flex-col overflow-hidden border border-border bg-card">
+            <div className="flex-1 flex flex-col overflow-hidden min-h-0 border border-border bg-card">
               <FileManagerSidebar
                 currentHost={currentHost}
                 currentPath={currentPath}