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/tsconfig.json

@@ -0,0 +1,4 @@
+{
+	"extends": "astro/tsconfigs/strict",
+	"include": ["src", ".astro/types.d.ts", "emdash-env.d.ts", "worker-configuration.d.ts"]
+}