Implement contact form with hCaptcha and update documentation

.nvmrc

@@ -0,0 +1 @@
+22

README.md

@@ -36,6 +36,21 @@ The actual pages are in [`src/pages/`](/src/pages/) and are mostly Markdown/MDX.
 
 ## Development Commands
 
+### Node version
+
+The repo ships an [`.nvmrc`](./.nvmrc), so with
+[nvm](https://github.com/nvm-sh/nvm) installed you can pick the right version automatically:
+
+```sh
+nvm install   # first time only – installs the version from .nvmrc
+nvm use       # switch the current shell to it
+```
+
+If `npm run build` fails with `Node.js vXX is not supported by Astro!`, you are on an
+older version – run `nvm use` and try again.
+
+### Commands
+
 All commands are run from the root of the project:
 
 | Command                   | Action                                           |
@@ -52,6 +67,28 @@ All commands are run from the root of the project:
 
 ---
 
+## Testing the contact form
+
+The contact forms ([`ContactForm.astro`](/src/components/ContactForm.astro)) use hCaptcha for
+spam protection, and **sending cannot be tested on a local dev server**:
+
+- hCaptcha refuses to run on `localhost` and `127.0.0.1`.
+- Its official test key renders the widget anywhere, but produces a token that only validates
+  against the matching test secret – which Web3Forms does not hold. Submitting then fails with
+  `Could not validate hCaptcha`.
+
+So test the full send on the deployed site. Locally you can still work on everything else by
+rendering the form without the captcha:
+
+```astro
+<ContactForm captcha={false} />
+```
+
+If a submission fails, the browser console carries the message Web3Forms actually returned;
+the visitor only ever sees the generic localised error.
+
+---
+
 ## Further Reading
 
 - [Astro documentation](https://docs.astro.build)