feat(markdown): add MarkdownSprite and register in SceneManager

Creates MarkdownSprite (Graphics-only background rect with header bar)
and wires it into SceneManager._createItem() under case 'markdown'.

frontend/src/canvas/sprites/MarkdownSprite.ts

@@ -0,0 +1,67 @@
+/**
+ * MarkdownSprite — PixiJS background container for markdown cards.
+ *
+ * Renders only a colored background rectangle for canvas integration
+ * (hit-testing, selection, z-ordering). All markdown content is rendered
+ * as a DOM overlay by MarkdownOverlay, not by PixiJS.
+ */
+
+import { Container, Graphics } from 'pixi.js';
+import type { MarkdownObject } from '../scene-format';
+import {
+  MD_DEFAULT_BG,
+  MD_DEFAULT_CORNER_RADIUS,
+  MD_HEADER_BG,
+} from '../markdownDefaults';
+
+const HEADER_HEIGHT = 32;
+
+export class MarkdownSprite extends Container {
+  private _bg: Graphics;
+  private _lastW = 0;
+  private _lastH = 0;
+  private _lastBgColor = '';
+  private _lastCornerRadius = 0;
+
+  constructor(data: MarkdownObject) {
+    super();
+    this._bg = new Graphics();
+    this.addChild(this._bg);
+    this._draw(data.w, data.h, data.bgColor || MD_DEFAULT_BG, data.cornerRadius ?? MD_DEFAULT_CORNER_RADIUS);
+  }
+
+  updateFromData(data: MarkdownObject): void {
+    const bgColor = data.bgColor || MD_DEFAULT_BG;
+    const cr = data.cornerRadius ?? MD_DEFAULT_CORNER_RADIUS;
+    if (data.w === this._lastW && data.h === this._lastH && bgColor === this._lastBgColor && cr === this._lastCornerRadius) {
+      return; // nothing changed
+    }
+    this._draw(data.w, data.h, bgColor, cr);
+  }
+
+  private _draw(w: number, h: number, bgColor: string, cornerRadius: number): void {
+    this._lastW = w;
+    this._lastH = h;
+    this._lastBgColor = bgColor;
+    this._lastCornerRadius = cornerRadius;
+
+    const g = this._bg;
+    g.clear();
+
+    // Card background
+    g.roundRect(0, 0, w, h, cornerRadius);
+    g.fill({ color: bgColor });
+
+    // Header bar
+    // Draw as a rect clipped to the top portion with rounded top corners
+    g.roundRect(0, 0, w, HEADER_HEIGHT, cornerRadius);
+    g.fill({ color: MD_HEADER_BG });
+    // Flat bottom edge of header: overwrite the bottom-rounded part
+    g.rect(0, HEADER_HEIGHT - cornerRadius, w, cornerRadius);
+    g.fill({ color: MD_HEADER_BG });
+
+    // Header bottom border
+    g.rect(0, HEADER_HEIGHT - 1, w, 1);
+    g.fill({ color: '#333333' });
+  }
+}