fix: couple mobile add-bar offset to container padding via CSS var

Previously .mobile-add-bar { margin-bottom: -80px } was hardcoded
to match .container { padding-bottom: 80px }. Now both reference
--m-container-pad-bottom so they can't drift out of sync.

public/css/mobile.css

@@ -70,7 +70,8 @@
     /* ── Container ──────────────────────────────────── */
 
     .container {
-        padding: 10px 12px 80px;
+        --m-container-pad-bottom: 80px;
+        padding: 10px 12px var(--m-container-pad-bottom);
         gap: 12px;
     }
 
@@ -598,7 +599,7 @@
         /* Negative margin to break out of container padding */
         margin-left: -12px;
         margin-right: -12px;
-        margin-bottom: -80px;
+        margin-bottom: calc(-1 * var(--m-container-pad-bottom));
     }
 
     .mobile-add-bar input {