fix(ui): mobile breakpoint layout and peek interactions

Grid forced to single-column at ≤900px for all panel states.
Resize handles hidden, transitions killed to prevent slivers.
Peek pane gets mobile toolbar (expand/dismiss buttons).
Escape dismisses peek at any viewport. Z toggles full-screen
peek at mobile instead of no-op zen toggle.

web/app.js

@@ -694,16 +694,21 @@
 
     pane.classList.add('visible');
 
+    const mobileBar = `<div class="peek-mobile-bar">
+      <button class="peek-mobile-btn" onclick="nibApp.togglePeekFull()">↑</button>
+      <button class="peek-mobile-btn" onclick="nibApp.dismissPeek()">×</button>
+    </div>`;
+
     if (state.peekMode === 'edit') {
-      pane.innerHTML = renderEditMode(e);
+      pane.innerHTML = mobileBar + renderEditMode(e);
     } else if (state.view === 'stream' || !e.card_type) {
-      pane.innerHTML = renderStreamPeek(e);
+      pane.innerHTML = mobileBar + renderStreamPeek(e);
     } else if (state.peekMode === 'run') {
-      pane.innerHTML = renderRunMode(e);
+      pane.innerHTML = mobileBar + renderRunMode(e);
     } else if (state.peekMode === 'fill') {
-      pane.innerHTML = renderFillMode(e);
+      pane.innerHTML = mobileBar + renderFillMode(e);
     } else {
-      pane.innerHTML = renderCardPeek(e);
+      pane.innerHTML = mobileBar + renderCardPeek(e);
     }
 
     bindPeekEvents(e);
@@ -1401,6 +1406,20 @@
       if (idx >= 0) { state.selectedIndex = idx; renderEntityList(); renderDetailPane(); }
       showToast(e.pinned ? 'unpinned' : 'pinned');
     },
+
+    togglePeekFull() {
+      const pane = $('#detail-pane');
+      pane.classList.toggle('peek-full');
+      const btn = pane.querySelector('.peek-mobile-btn');
+      if (btn) btn.textContent = pane.classList.contains('peek-full') ? '↓' : '↑';
+    },
+
+    dismissPeek() {
+      const pane = $('#detail-pane');
+      pane.classList.remove('visible', 'peek-full');
+      state.selectedIndex = -1;
+      renderEntityList();
+    },
   };
 
   // ========== Promote modal ==========
@@ -1451,12 +1470,18 @@
       return;
     }
 
-    if (ev.key === 'Escape' && $('main').classList.contains('focus-peek')) {
-      exitFocusPeek();
-      state.selectedIndex = -1;
-      renderEntityList();
-      renderDetailPane();
-      return;
+    if (ev.key === 'Escape') {
+      const pane = $('#detail-pane');
+      if ($('main').classList.contains('focus-peek')) {
+        exitFocusPeek();
+      }
+      if (pane.classList.contains('visible')) {
+        pane.classList.remove('visible', 'peek-full');
+        state.selectedIndex = -1;
+        renderEntityList();
+        renderDetailPane();
+        return;
+      }
     }
 
     const sel = state.entities[state.selectedIndex];
@@ -1540,7 +1565,16 @@
     localStorage.setItem('nib:' + cls, m.classList.contains(cls) ? '1' : '');
   }
 
+  function isMobileBreakpoint() {
+    return window.matchMedia('(max-width: 900px)').matches;
+  }
+
   function toggleZen() {
+    if (isMobileBreakpoint()) {
+      if (state.selectedIndex >= 0) nibApp.togglePeekFull();
+      return;
+    }
+
     const m = $('main');
 
     if (m.classList.contains('focus-peek')) {