feat: add SPC w {h,j,k,l} spatial window navigation

Pane focus follows actual screen layout: Artists (left), Albums (top-right),
Tracks (bottom-right). h/l jump between left/right columns, j/k move
between vertically stacked panes in the right column.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/claude-agent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/input/action.rs

@@ -5,7 +5,8 @@ pub enum AppAction {
     MoveDown,
     FocusLeft,
     FocusRight,
-    CycleFocus,
+    FocusDown,
+    FocusUp,
     GotoFirst,
     GotoLast,
     HalfPageDown,