feat: add tabbed grids, click-to-expand, and keybind cleanup

Introduces per-tab pane management with independent focus/expansion state,
a persistent tab bar visible in both picker and grid modes, and a 70/30
soft-expand layout triggered by clicking pane bodies. Removes legacy
toggle keybinds (Ctrl+^ and Ctrl+`) leaving only Ctrl+Space.

New keybinds: Ctrl+T new tab, Alt+1-9 switch tab, Alt+n/p cycle tabs,
Ctrl+E toggle click-expand, Ctrl+W auto-removes empty tabs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/index.ts

@@ -81,6 +81,12 @@ async function main() {
     height: "100%",
   })
 
+  app.tabBarText = new TextRenderable(app.renderer, {
+    width: "100%",
+    height: 1,
+    flexShrink: 0,
+  })
+
   app.headerText = new TextRenderable(app.renderer, {
     width: "100%",
     height: 1,
@@ -148,6 +154,7 @@ async function main() {
     flexShrink: 0,
   })
 
+  app.mainBox.add(app.tabBarText)
   app.mainBox.add(app.headerText)
   app.mainBox.add(app.colHeaderText)
   app.mainBox.add(app.listBox)