Emdash source with visual editor image upload fix

Fixes:
1. media.ts: wrap placeholder generation in try-catch
2. toolbar.ts: check r.ok, display error message in popover

packages/core/tests/unit/media/thumbnail.test.ts

@@ -0,0 +1,57 @@
+import { describe, expect, it } from "vitest";
+
+import { THUMBNAIL_MAX_DIMENSION, computeThumbnailSize } from "../../../src/media/thumbnail.js";
+
+describe("computeThumbnailSize", () => {
+	it("scales a square image to the max dimension", () => {
+		expect(computeThumbnailSize(5000, 5000)).toEqual({
+			width: THUMBNAIL_MAX_DIMENSION,
+			height: THUMBNAIL_MAX_DIMENSION,
+		});
+	});
+
+	it("scales a wide image to fit within the bounding box", () => {
+		const result = computeThumbnailSize(4000, 2000);
+		expect(result.width).toBe(THUMBNAIL_MAX_DIMENSION);
+		expect(result.height).toBe(THUMBNAIL_MAX_DIMENSION / 2);
+	});
+
+	it("scales a tall image to fit within the bounding box", () => {
+		const result = computeThumbnailSize(2000, 4000);
+		expect(result.width).toBe(THUMBNAIL_MAX_DIMENSION / 2);
+		expect(result.height).toBe(THUMBNAIL_MAX_DIMENSION);
+	});
+
+	it("clamps extreme tall aspect ratios to the bounding box", () => {
+		// Without clamping, naive code would produce a 64×537600 canvas.
+		const result = computeThumbnailSize(100, 840_000);
+		expect(result.width).toBeLessThanOrEqual(THUMBNAIL_MAX_DIMENSION);
+		expect(result.height).toBeLessThanOrEqual(THUMBNAIL_MAX_DIMENSION);
+		expect(result.width).toBeGreaterThanOrEqual(1);
+		expect(result.height).toBe(THUMBNAIL_MAX_DIMENSION);
+	});
+
+	it("clamps extreme wide aspect ratios to the bounding box", () => {
+		const result = computeThumbnailSize(840_000, 100);
+		expect(result.width).toBe(THUMBNAIL_MAX_DIMENSION);
+		expect(result.height).toBeGreaterThanOrEqual(1);
+		expect(result.height).toBeLessThanOrEqual(THUMBNAIL_MAX_DIMENSION);
+	});
+
+	it("never upscales smaller images", () => {
+		expect(computeThumbnailSize(10, 20)).toEqual({ width: 10, height: 20 });
+		expect(computeThumbnailSize(1, 1)).toEqual({ width: 1, height: 1 });
+	});
+
+	it("returns a 1x1 fallback for zero or negative dimensions", () => {
+		expect(computeThumbnailSize(0, 100)).toEqual({ width: 1, height: 1 });
+		expect(computeThumbnailSize(100, 0)).toEqual({ width: 1, height: 1 });
+		expect(computeThumbnailSize(-5, 10)).toEqual({ width: 1, height: 1 });
+	});
+
+	it("rounds fractional dimensions", () => {
+		const result = computeThumbnailSize(300, 199);
+		expect(Number.isInteger(result.width)).toBe(true);
+		expect(Number.isInteger(result.height)).toBe(true);
+	});
+});