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).

app/globals.css

@@ -127,3 +127,33 @@ body {
 .dock-variant {
 	animation: dock-variant-in 160ms cubic-bezier(0.2, 0.8, 0.2, 1);
 }
+
+/* ============================================================
+ * W3 — presentation fixes (appended 2026-08-22)
+ * ============================================================ */
+
+/* Streamdown's internal UI chrome (code blocks, tables, mermaid containers,
+   copy/download buttons) is styled with Tailwind utilities living in its dist
+ * bundle. Tailwind v4 never scans node_modules, so without this line those
+ * classes are dead — code blocks and mermaid containers rendered unstyled.
+ * (Streamdown README, "Tailwind CSS setup".) */
+@source "../node_modules/streamdown/dist/*.js";
+
+/* Token the streamdown utilities reference (code-block/mermaid chrome uses
+ * bg-sidebar / border-sidebar): map it onto the app's card surface. */
+@theme inline {
+	--color-sidebar: var(--card);
+}
+
+/* Mermaid diagrams: keep wide charts inside the content column. Mermaid
+ * inlines a px max-width on the svg, so !important is required to win. */
+[data-streamdown="mermaid"] svg {
+	max-width: 100% !important;
+	height: auto !important;
+}
+
+/* Display math: long equations scroll instead of overflowing the column. */
+.katex-display {
+	overflow-x: auto;
+	overflow-y: hidden;
+}