#260: .swapbtn grid for copy feedback (fix attempt 2)

Label and checkmark stack in one grid cell (inline-grid, grid-area 1/1,
checkmark visibility-hidden until .ok), so the button width is static by
construction across the click. Replaces the min-width pin from fix attempt
1, which enlarged buttons by up to 0.64px (ceil of fractional width) and
leaked the pin after restore.

Applies to paste-view Copy/Link (paste.html + paste.js copyFeedback) and
the /new result-box copy button (new.js). Verified with CDP bounding-box
probes: Copy width/x identical before/during/after at 1400x900 and
375x812 (including the previously failing 52.36px mobile case), Link
button static, checkmark visible during feedback, no minWidth pin set.

internal/web/static/app.css

@@ -580,6 +580,14 @@ a.admin-link:hover { color: var(--fg); text-decoration: underline; }
   outline-offset: 1px;
 }
 
+/* swapbtn (#260): the label and the checkmark occupy the same grid cell, so
+   toggling feedback never changes the button's width — no pinning needed */
+.swapbtn { display: inline-grid; }
+.swapbtn > * { grid-area: 1 / 1; }
+.swapbtn .btn-check { visibility: hidden; }
+.swapbtn.ok .btn-check { visibility: visible; }
+.swapbtn.ok .btn-label { visibility: hidden; }
+
 /* in-place copy success feedback (#53) */
 .iconbtn.ok, .btn.ok {
   color: var(--ok);