#260 (fix attempt 2): static-width copy feedback via .swapbtn

internal/web/static/app.css

@@ -586,6 +586,20 @@ a.admin-link:hover { color: var(--fg); text-decoration: underline; }
   border-color: var(--ok);
 }
 
+/* #260: static-width success feedback. Label and checkmark stack in one
+   grid cell, so the button is always as wide as the wider of the two and
+   never shifts on click. Feedback is a pure .ok class toggle. */
+.swapbtn {
+  display: inline-grid;
+}
+.swapbtn > * {
+  grid-area: 1 / 1;
+  justify-self: center;
+}
+.swapbtn .swap-check { visibility: hidden; }
+.swapbtn.ok .swap-check { visibility: visible; }
+.swapbtn.ok .swap-label { visibility: hidden; }
+
 /* headings: unified treatment (mirrors .side-section h3) */
 .settings-head h1, .paste-title-bar h1, .head-row h1, .inner h1 {
   letter-spacing: -0.01em;