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/blocks/playground/wrangler.jsonc

@@ -0,0 +1,4 @@
+{
+	"name": "emdash-blocks",
+	"compatibility_date": "2026-02-25",
+}