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

demos/preview/wrangler.jsonc

@@ -0,0 +1,27 @@
+{
+	"$schema": "node_modules/wrangler/config-schema.json",
+	"name": "emdash-preview",
+	// Custom entrypoint that exports EmDashPreviewDB
+	"main": "./src/worker.ts",
+	"compatibility_date": "2026-01-14",
+	"compatibility_flags": ["nodejs_compat", "disable_nodejs_process_v2"],
+	// Durable Object for preview databases
+	"durable_objects": {
+		"bindings": [
+			{
+				"name": "PREVIEW_DB",
+				"class_name": "EmDashPreviewDB",
+			},
+		],
+	},
+	// DO migration — declares the SQLite-backed class
+	"migrations": [
+		{
+			"tag": "v1",
+			"new_sqlite_classes": ["EmDashPreviewDB"],
+		},
+	],
+	"observability": {
+		"enabled": true,
+	},
+}