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/types/ui-types.ts

@@ -272,6 +272,7 @@ export type SplitMode =
   | "none"
   | "2-way"
   | "3-way"
+  | "3-way-horizontal"
   | "4-way"
   | "5-way"
   | "6-way";