feat(markdown): add markdownDefaults module with constants and extractTitle

frontend/src/canvas/markdownDefaults.ts

@@ -0,0 +1,18 @@
+// markdownDefaults.ts — default values and constraints for markdown blocks
+
+export const MD_DEFAULT_WIDTH = 450;
+export const MD_MIN_WIDTH = 250;
+export const MD_MAX_WIDTH = 800;
+export const MD_DEFAULT_BG = '#232336';
+export const MD_DEFAULT_TEXT_COLOR = '#e0e0e0';
+export const MD_DEFAULT_ACCENT = '#7950f2';
+export const MD_DEFAULT_PADDING = 16;
+export const MD_DEFAULT_CORNER_RADIUS = 10;
+export const MD_HEADER_BG = '#2a2a42';
+export const MD_MAX_CONTENT_LENGTH = 50_000;
+
+/** Extract the first H1 from markdown content for the card header title. */
+export function extractTitle(content: string): string {
+  const match = content.match(/^#\s+(.+)$/m);
+  return match ? match[1].trim() : 'Untitled';
+}