fix: Theme App Extension only allows assets/blocks/snippets/locales dirs

`shopify app dev` failed dev preview with "Only assets, blocks, snippets,
locales directories are allowed" — the widget's TypeScript source lived in
extensions/datetime-widget/src/, which isn't one of the four directories a
Theme App Extension may contain. Moved it to widget-src/datetime-widget/
(a plain, non-extension folder outside extensions/) and updated
build:widget's esbuild input path accordingly; the bundled output still
lands in the same place (extensions/datetime-widget/assets/).

Also fixed a theme-check warning surfaced during the same run:
`script_tag` renders a parser-blocking <script> with no way to defer it —
switched to a manual <script defer> tag for the widget's JS asset.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -45,8 +45,13 @@ slot-holds) are complete. See §6 of `IMPLEMENTATION_PLAN.md` for the phased
 build order and acceptance criteria — next up is Phase 5 (multi-location,
 zones, rates, auto-assignment).
 
-Editing the storefront widget's `extensions/datetime-widget/src/`? Run
-`npm run build:widget` to rebuild `assets/datetime-widget.js` — it also runs
+The storefront widget's TypeScript source lives in `widget-src/datetime-widget/`,
+**not** inside `extensions/datetime-widget/` — a Theme App Extension's
+directory may only contain `assets`, `blocks`, `snippets`, and `locales`
+(the CLI hard-rejects anything else, e.g. a `src/` folder, with "Only
+assets, blocks, snippets, locales directories are allowed"). Editing the
+widget? Run `npm run build:widget` to bundle it into
+`extensions/datetime-widget/assets/datetime-widget.js` — it also runs
 automatically before `npm run dev` / `npm run deploy`.
 
 **Functions are JavaScript, not Rust** (`extensions/validation-slot/`,

extensions/datetime-widget/blocks/app-embed.liquid

@@ -1,5 +1,5 @@
 {{ 'datetime-widget.css' | asset_url | stylesheet_tag }}
-{{ 'datetime-widget.js' | asset_url | script_tag }}
+<script src="{{ 'datetime-widget.js' | asset_url }}" defer="defer"></script>
 
 {% schema %}
 {

package.json

@@ -22,7 +22,7 @@
     "test": "vitest",
     "test:integration": "vitest run --config vitest.integration.config.ts",
     "test:e2e": "playwright test",
-    "build:widget": "esbuild extensions/datetime-widget/src/datetime-widget.ts --bundle --minify --target=es2019 --outfile=extensions/datetime-widget/assets/datetime-widget.js",
+    "build:widget": "esbuild widget-src/datetime-widget/datetime-widget.ts --bundle --minify --target=es2019 --outfile=extensions/datetime-widget/assets/datetime-widget.js",
     "worker": "tsx jobs/worker.ts",
     "shopify": "shopify",
     "prisma": "prisma",