fix(ux): scope mermaid min-height to inline+mobile only

Remove min-height:200px from the global .mermaid-viewer-viewport rule
(it was outside the @media(max-width:640px) block despite the PR title).
Add a properly-scoped .mermaid-viewer--inline .mermaid-viewer-viewport
min-height inside an actual @media(max-width:640px) block, capped at
min(200px, 70vh) so it cannot override the lightbox fit-to-screen envelope.

static/style.css

@@ -2396,9 +2396,12 @@
   .mermaid-viewer-btn{width:28px;height:28px;border:1px solid var(--border2);border-radius:6px;background:var(--input-bg);color:var(--muted);display:inline-flex;align-items:center;justify-content:center;cursor:pointer;transition:background .15s,color .15s,border-color .15s,transform .15s;flex:0 0 auto;padding:0;}
   .mermaid-viewer-btn:hover{background:var(--hover-bg);color:var(--text);}
   .mermaid-viewer-btn svg{width:14px;height:14px;display:block;fill:none;stroke:currentColor;stroke-width:1.8;stroke-linecap:round;stroke-linejoin:round;}
-  .mermaid-viewer-viewport{position:relative;overflow:hidden;max-width:100%;min-height:200px;border:1px solid var(--border);border-radius:8px;background:var(--code-bg);cursor:grab;touch-action:none;user-select:none;-webkit-user-select:none;}
+  .mermaid-viewer-viewport{position:relative;overflow:hidden;max-width:100%;border:1px solid var(--border);border-radius:8px;background:var(--code-bg);cursor:grab;touch-action:none;user-select:none;-webkit-user-select:none;}
   .mermaid-viewer-viewport.is-panning{cursor:grabbing;}
   .mermaid-viewer--inline .mermaid-viewer-viewport{width:100%;max-height:70vh;}
+  @media (max-width: 640px){
+    .mermaid-viewer--inline .mermaid-viewer-viewport{min-height:min(200px,70vh);}
+  }
   .mermaid-viewer--lightbox .mermaid-viewer-viewport{max-width:90vw;max-height:90vh;border:none;box-shadow:0 8px 48px rgba(0,0,0,.6);}
   .mermaid-viewer-canvas{position:absolute;left:0;top:0;transform-origin:0 0;will-change:transform;}
   .mermaid-viewer-svg{display:block;width:100%;height:100%;}