feat(ui): context-sensitive z key (focus mode)

- Nothing selected: z toggles zen (hide both panels)
- Item selected: z expands peek to full width (focus mode)
- z again or Esc exits focus mode and deselects
- j/k still cycle items while in focus mode

web/app.js

@@ -1451,6 +1451,14 @@
       return;
     }
 
+    if (ev.key === 'Escape' && $('main').classList.contains('focus-peek')) {
+      exitFocusPeek();
+      state.selectedIndex = -1;
+      renderEntityList();
+      renderDetailPane();
+      return;
+    }
+
     const sel = state.entities[state.selectedIndex];
 
     switch (ev.key) {
@@ -1534,6 +1542,17 @@
 
   function toggleZen() {
     const m = $('main');
+
+    if (m.classList.contains('focus-peek')) {
+      exitFocusPeek();
+      return;
+    }
+
+    if (state.selectedIndex >= 0) {
+      m.classList.add('focus-peek');
+      return;
+    }
+
     const isZen = m.classList.contains('hide-rail') && m.classList.contains('hide-peek');
     if (isZen) {
       m.classList.remove('hide-rail', 'hide-peek');
@@ -1546,6 +1565,10 @@
     }
   }
 
+  function exitFocusPeek() {
+    $('main').classList.remove('focus-peek');
+  }
+
   (function restorePanels() {
     const m = $('main');
     if (localStorage.getItem('nib:hide-rail')) m.classList.add('hide-rail');