W3: presentation fixes — delimiters, line breaks, mermaid

- src/core/math-delims.ts: pure preprocessing for the \(...\)/\[...\] math
  contract — balanced delimiters rewrite to the two-dollar forms remark-math
  parses ($ is never math). Skips code fences/spans, escaped delimiters, and
  leaves unbalanced (mid-stream) delimiters untouched. Display math renders
  as a block: mid-paragraph gets blank-line padding; indented (list items)
  degrades to inline so list continuations don't half-parse.
- message.tsx: transform in the render path (children-memoized); math plugin
  now singleDollarTextMath: false — the currency hazard dies structurally.
  remark-breaks via the cjk remarkPluginsAfter slot (the bare remarkPlugins
  prop would replace the [remark-gfm, codeMeta] defaults).
- mermaid: dark theme + sans font via Streamdown's mermaid prop; dark-styled
  error fallback (components/mermaid-error.tsx) with error, source in
  <details>, and retry.
- globals.css (appended): @source for streamdown dist — its internal UI
  chrome (code blocks, mermaid containers, tables) was entirely unstyled
  because Tailwind v4 never scans node_modules; --color-sidebar → card; wide
  mermaid SVGs capped to the column; .katex-display scrolls.
- tests/render-markdown.test.tsx: 26 tests (transform unit + render). The
  math describe block in render-cards.test.tsx is updated to the new
  contract ($...$ now renders as literal text — pinned).

components/mermaid-error.tsx

@@ -0,0 +1,44 @@
+"use client";
+
+import { Button } from "@/components/ui/button";
+import { cn } from "@/lib/utils";
+import type { MermaidErrorComponentProps } from "streamdown";
+
+/**
+ * Fallback for mermaid diagrams that fail to parse — wired via Streamdown's
+ * `mermaid.errorComponent`. Replaces the default light-themed red box with a
+ * card matching the app: the error, the source in a details block (so a
+ * broken diagram doesn't flood the transcript), and a retry.
+ */
+export const MermaidError = ({
+	chart,
+	error,
+	retry,
+	className,
+}: MermaidErrorComponentProps & { className?: string }) => (
+	<div
+		className={cn(
+			"my-4 rounded-xl border border-destructive/40 bg-destructive/5 p-4",
+			className,
+		)}
+		data-streamdown="mermaid-error"
+	>
+		<p className="text-sm font-medium text-destructive">
+			Diagram failed to render
+		</p>
+		<p className="mt-1 font-mono text-xs break-words text-muted-foreground">
+			{error}
+		</p>
+		<details className="mt-3">
+			<summary className="cursor-pointer text-xs text-muted-foreground">
+				Show diagram source
+			</summary>
+			<pre className="mt-2 overflow-x-auto rounded-lg bg-muted p-3 font-mono text-xs text-foreground">
+				{chart}
+			</pre>
+		</details>
+		<Button className="mt-3" onClick={retry} size="sm" variant="outline">
+			Retry
+		</Button>
+	</div>
+);