fix(preview): pin SVG to intrinsic size for all diagram types

The previous fix only disabled useMaxWidth for flowcharts, so sequence (and
other) diagrams still rendered responsively and fit shrank them to a tiny
speck. Normalise every rendered SVG to its viewBox pixel size (stripping
width:100% / max-width) in the render path, so fit/zoom is exact regardless of
diagram type. Drops the now-redundant per-type flowchart flag.

src/lib/mermaid.ts

@@ -15,10 +15,37 @@ function configure(theme: string) {
     theme: theme as any,
     securityLevel: 'loose',
     fontFamily: 'inherit',
-    // Render at the diagram's intrinsic pixel size (not responsive 100% width)
+    flowchart: { htmlLabels: true },
-    // so the preview's own zoom/fit maths stay exact — otherwise a large
+  });
-    // flowchart is scaled down by the SVG *and* again by fit, ending up tiny.
+}
-    flowchart: { useMaxWidth: false, htmlLabels: true },
+
+/**
+ * Force an SVG to its intrinsic pixel size (from the viewBox), stripping the
+ * responsive `width:100%` / `max-width` that Mermaid emits by default.
+ *
+ * Mermaid's `useMaxWidth` makes diagrams shrink to their container, but the
+ * preview applies its own zoom/fit transform — so a responsive SVG ends up
+ * scaled down twice and renders tiny. Pinning the laid-out size to the viewBox
+ * keeps fit/zoom exact for *every* diagram type (sequence, class, …), which
+ * per-type `useMaxWidth` flags can't guarantee uniformly.
+ */
+function pinSize(svg: string): string {
+  const vb = svg.match(/viewBox="([\d.\s-]+)"/);
+  if (!vb) return svg;
+  const p = vb[1].trim().split(/\s+/).map(Number);
+  if (p.length !== 4 || p.some(Number.isNaN)) return svg;
+  const [, , w, h] = p;
+  return svg.replace(/<svg\b[^>]*>/, (tag) => {
+    let t = tag.replace(/\s(?:width|height)="[^"]*"/g, '');
+    t = t.replace(/\sstyle="([^"]*)"/, (_m, s: string) => {
+      const cleaned = s
+        .split(';')
+        .map((d) => d.trim())
+        .filter((d) => d && !/^(?:max-width|width)\s*:/i.test(d))
+        .join('; ');
+      return cleaned ? ` style="${cleaned}"` : '';
+    });
+    return t.replace(/<svg\b/, `<svg width="${w}" height="${h}"`);
   });
 }
 
@@ -33,7 +60,7 @@ export async function renderMermaid(code: string, theme: string): Promise<Render
   const id = `mmx-render-${++renderSeq}`;
   try {
     const { svg } = await mermaid.render(id, code);
-    return { svg };
+    return { svg: pinSize(svg) };
   } catch (e) {
     return { error: e instanceof Error ? e.message : String(e) };
   } finally {