Implement fit-mode rendering contract with container sizing and pane re-fit

src/app/components/LivePreview.module.css

@@ -1,16 +1,114 @@
 .preview {
+  display: flex;
+  flex-direction: column;
   height: 100%;
   width: 100%;
-  overflow: auto;
   background: var(--bg);
 }
 
-.chart {
+.header {
+  flex: 0 0 auto;
   display: flex;
-  align-items: flex-start;
-  justify-content: center;
-  min-height: 100%;
+  align-items: center;
+  justify-content: flex-end;
+  gap: var(--space-3);
+  height: 40px;
+  padding: 0 var(--space-4);
+  border-bottom: var(--border-width) solid var(--border);
+}
+
+.body {
+  flex: 1 1 auto;
+  min-height: 0;
+  overflow: auto;
   padding: var(--space-5);
+  box-sizing: border-box;
+}
+
+/* Segmented control — the four fit modes (spec §04). */
+.fit {
+  display: inline-flex;
+  border: var(--border-width) solid var(--border-strong);
+}
+
+.fitOption {
+  appearance: none;
+  border: none;
+  background: var(--bg);
+  color: var(--text-secondary);
+  font: inherit;
+  font-size: 12px;
+  line-height: 1;
+  padding: var(--space-2) var(--space-3);
+  cursor: pointer;
+  transition:
+    background var(--dur-fast) var(--ease),
+    color var(--dur-fast) var(--ease);
+}
+
+.fitOption + .fitOption {
+  border-left: var(--border-width) solid var(--border-strong);
+}
+
+.fitOption:hover {
+  background: var(--layer-01);
+  color: var(--text);
+}
+
+.fitActive,
+.fitActive:hover {
+  background: var(--accent);
+  color: var(--accent-contrast);
+}
+
+/*
+ * Chart sizing. The host (passed to vega-embed) is branded `.vega-embed`
+ * (display:inline-block) at runtime; that shrink-wraps it, which is why
+ * width:"container" collapsed before. The frame carries the fit class and the
+ * two-class selectors below out-specify `.vega-embed` to give the host a
+ * definite box for the responsive modes. `box-sizing:border-box` keeps the
+ * chart inside the body padding rather than overflowing it.
+ */
+.frame {
+  box-sizing: border-box;
+}
+
+.host {
+  box-sizing: border-box;
+}
+
+/* Original — natural size; the body scrolls if the chart is larger than the pane. */
+.fitOriginal {
+  display: inline-block;
+}
+
+/* Width — host spans the pane width; height stays natural. */
+.fitWidth {
+  display: block;
+  width: 100%;
+}
+.fitWidth .host {
+  width: 100%;
+}
+
+/* Height — host spans the pane height; width stays natural. */
+.fitHeight {
+  display: block;
+  height: 100%;
+}
+.fitHeight .host {
+  height: 100%;
+}
+
+/* Full — host fills the pane in both dimensions. */
+.fitFull {
+  display: block;
+  width: 100%;
+  height: 100%;
+}
+.fitFull .host {
+  width: 100%;
+  height: 100%;
 }
 
 .error {