feat(site-builder): tablet/mobile style overrides that never touch desktop

On tablet/mobile the useNodeProp style setters now route responsive keys
(spacing, font size, line height, letter spacing, alignment, size, flex
layout) into styleTablet/styleMobile; colours and other keys still write
style. An empty value removes the override. Continuous-burst keys include
the device so a desktop drag and a mobile drag never merge.

- shared.tsx: device-aware setters, new setResponsiveStyleProp /
  clearResponsiveStyleProp exports, setPropStylesContinuous
- responsive-panel.tsx: useResolvedStyle, useOverrideState, DeviceBanner
- OverrideDot: filled dot resets the override (one undo step), hollow dot
  marks an inherited value; wired into Text/Button/Container/Image/Media/
  BackgroundSection panels and both box-model sections
- Columns: "Stack on this device" toggle (flexDirection override)
- GuidedStyles: device banner under the type header
- RenderNode: every component node renders through one stable
  ResolvedRender so gaining/losing an override or switching device no
  longer remounts the node (the remount dropped inline-edit focus/caret
  and blanked a selected Heading)

Refs #45

craft/src/styles/editor.css

@@ -2441,3 +2441,72 @@ body {
   background: var(--color-accent-subtle);
   cursor: default;
 }
+
+/* #45 overrides */
+.wsb-override-dot {
+  display: inline-block;
+  width: 8px;
+  height: 8px;
+  margin-left: 6px;
+  padding: 0;
+  vertical-align: middle;
+  border-radius: 50%;
+  box-sizing: border-box;
+}
+
+.wsb-override-dot.is-set {
+  background: var(--color-accent);
+  border: 1px solid var(--color-accent);
+  cursor: pointer;
+}
+
+.wsb-override-dot.is-set:hover,
+.wsb-override-dot.is-set:focus-visible {
+  background: var(--color-danger);
+  border-color: var(--color-danger);
+  outline: none;
+}
+
+.wsb-override-dot.is-inherited {
+  background: transparent;
+  border: 1px solid var(--color-text-dim);
+}
+
+.wsb-device-banner {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 6px 8px;
+  margin: 0 0 8px;
+  padding: 8px 10px;
+  font-size: 11px;
+  line-height: 1.4;
+  color: var(--color-text);
+  background: var(--color-accent-subtle);
+  border: 1px solid var(--color-accent);
+  border-radius: var(--radius-md);
+}
+
+.wsb-device-banner > span {
+  flex: 1 1 180px;
+}
+
+.wsb-device-banner-back {
+  padding: 0;
+  font: inherit;
+  font-weight: 600;
+  color: var(--color-accent);
+  background: none;
+  border: 0;
+  cursor: pointer;
+  text-decoration: underline;
+}
+
+.wsb-stack-toggle {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  font-size: 11px;
+  color: var(--color-text);
+  cursor: pointer;
+}