fix: clamp the desktop container widths to the available space

The desktop band widths introduced in #4869 are not tied to the
breakpoints that open them: the hd band starts at 1100px but sets a
1200px width, so every viewport from 1100px to 1199px got a container up
to 100px wider than the screen.

That does not surface as a horizontal scrollbar. `body` hides the
overflow and `margin-left/right: auto` cannot resolve to a negative
value, so the container is pinned to the left and its right edge is
silently clipped -- the layout stops being centred and part of the
header runs off-screen.

Each band is now clamped with `min(..., 100%)`. The hd band goes fluid
between 1100px and 1199px and pins to 1200px from 1200px up, as
intended. The xl/xxl bands are not affected today, but they are clamped
too: the widths are custom properties so a theme can retune them from
:root and reintroduce the same mismatch.

`100%` rather than `100vw`, because `body` has `overflow-y: scroll` and
100vw would include the scrollbar gutter. It also behaves correctly when
the discussion list pane is pinned, where the containing block is
already narrowed by the pane width.

Author: imorland

framework/core/less/common/scaffolding.less

@@ -68,14 +68,20 @@ p {
   // same fixed width, wasting half the viewport on modern displays. The widths
   // are CSS custom properties (see root.less) so themes can retune each band
   // without re-declaring the media queries.
+  //
+  // A band's width is not tied to the breakpoint that opens it, and a theme can
+  // set it to anything, so each is clamped to the viewport: a width wider than
+  // the band's own lower bound would otherwise overflow (body hides the overflow
+  // and auto margins cannot go negative, so the layout silently clips off-centre
+  // rather than scrolling).
   @media @desktop-hd {
-    width: var(--container-hd);
+    width: ~"min(var(--container-hd), 100%)";
   }
   @media @desktop-xl {
-    width: var(--container-xl);
+    width: ~"min(var(--container-xl), 100%)";
   }
   @media @desktop-xxl {
-    width: var(--container-xxl);
+    width: ~"min(var(--container-xxl), 100%)";
   }
 }