Fix header popover hanging off-screen on narrow viewports

The header wraps under 860px, so a menu-wrap can land at the start of a row.
`.popover` is right-anchored to its button, so the theme/language/account menu
rendered at x=-280 of a 390px screen — half of it off the left edge and its
buttons unclickable. Anchor the popover to the full-width header on narrow
screens instead.

Measured at 390x844 before/after: popover x=-280 → x=56, w=326, fully inside.

docker/frontend/src/styles/app.css

@@ -192,6 +192,13 @@ input[type="range"] { width: 100%; accent-color: var(--accent); }
 /* --- responsive --------------------------------------------------------- */
 @media (max-width: 860px) {
   .workspace { flex-direction: column; }
+  /* The header wraps on narrow screens, so a menu-wrap can land at the start of
+     a row — the popover is right-anchored to its button and would hang off the
+     left of the screen (measured: x=-280 of 390px). Anchor it to the full-width
+     header instead. */
+  .header { position: relative; }
+  .menu-wrap { position: static; }
+  .popover { right: 8px; max-width: calc(100vw - 16px); }
   .rail {
     order: 2;
     width: auto;