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 ==========
@@ -1451,13 +1470,19 @@
       return;
     }
 
-    if (ev.key === 'Escape' && $('main').classList.contains('focus-peek')) {
+    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')) {

web/style.css

@@ -785,6 +785,28 @@ main.focus-peek .resize-handle { visibility: hidden; }
     overflow: hidden;
 }
 
+.peek-mobile-bar {
+    display: none;
+    justify-content: space-between;
+    padding: 6px 12px;
+    border-bottom: 1px solid var(--border);
+}
+
+.peek-mobile-btn {
+    font-family: var(--mono);
+    font-size: 14px;
+    color: var(--muted);
+    background: none;
+    border: 1px solid var(--border);
+    border-radius: 4px;
+    width: 28px;
+    height: 28px;
+    cursor: pointer;
+    transition: color var(--t-fast), border-color var(--t-fast);
+}
+
+.peek-mobile-btn:hover { color: var(--accent); border-color: var(--accent); }
+
 .peek-scroll {
     flex: 1;
     overflow-y: auto;
@@ -1443,8 +1465,17 @@ kbd { background: var(--raised); border: 1px solid var(--border); border-radius:
 
 /* ── RESPONSIVE ─────────────────────────────────────── */
 @media (max-width: 900px) {
-    main { grid-template-columns: 1fr; }
-    #tag-rail { display: none; }
+    main,
+    main.hide-rail,
+    main.hide-peek,
+    main.hide-rail.hide-peek,
+    main.focus-peek {
+        grid-template-columns: 1fr !important;
+        transition: none !important;
+    }
+    #tag-rail { display: none !important; }
+    .resize-handle { display: none !important; }
+    #entity-panel { overflow: auto; }
     #detail-pane {
         position: fixed;
         inset: 0;
@@ -1458,4 +1489,7 @@ kbd { background: var(--raised); border: 1px solid var(--border); border-radius:
         z-index: 50;
     }
     #detail-pane.visible { transform: translateY(0); }
+    #detail-pane.peek-full { height: 100vh; height: 100dvh; top: 0; }
+    .peek-mobile-bar { display: flex; }
+    main.focus-peek #entity-panel { display: block; overflow: auto; min-width: 0; }
 }