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/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; }
 }