feat(markdown): set the preview font size apart from the UI font (#1045)

The Markdown preview had a fixed 14px body and notebook Markdown cells inherited the UI font size, so enlarging them meant scaling the whole UI. Languages & Frameworks › Markdown › Preview font size (application-level, 8–72, following the editor font size until set) now drives the preview and notebook Markdown cells; fenced code scales with it, and notebook code cells and outputs follow the editor font.

Fixes #768

Co-authored-by: jaw52 <[email protected]>

src/features/settings/languages/MarkdownPage.tsx

@@ -0,0 +1,30 @@
+import { useId } from "react";
+import { NumberInput, Row, Section } from "../components/Form";
+import { useSetting } from "../components/context";
+
+/** Languages & Frameworks › Markdown, after IDEA's: application-level, preview font size follows the editor font until set. */
+export function MarkdownPage() {
+  const id = useId();
+  const [size, setSize] = useSetting<number | null>("languages.markdown.previewFontSize");
+  const [editorSize] = useSetting<number>("editor.fontSize");
+  return (
+    <Section title="Preview">
+      <Row
+        label="Preview font size"
+        htmlFor={id}
+        hint="Also used by Markdown cells in Jupyter notebooks. Defaults to the editor font size, independent of the UI font size."
+      >
+        <NumberInput
+          id={id}
+          value={size ?? editorSize}
+          onChange={setSize}
+          min={8}
+          max={72}
+          integer
+          unit="px"
+          aria-label="Preview font size"
+        />
+      </Row>
+    </Section>
+  );
+}