Add contact actions and migrate contact form to Astro actions

Implement send_otp and send_msg handlers with server-side validation,
captcha verification, OTP generation/verification, rate limiting,
session-backed multi-step flow, and SMS gateway integration.

Replace broken contact endpoint and types with Astro server actions;
delete endpoints/contact.ts, types/ContactForm.ts and
src/components/ContactForm.astro. Update pages/contact.astro to use
astro:actions and improve captcha handling (listen for solve and set
hidden input). Adjust OTP timing (step 60s → 300s, VALID_PAST_OTP_STEPS
5 → 1). Add validator, @types/validator, Prettier and
prettier-plugin-astro; include .prettierrc.mjs in tsconfig include.

Add Prettier config for Astro

.prettierrc.mjs

@@ -0,0 +1,12 @@
+/** @type {import("prettier").Config} */
+export default {
+  plugins: ["prettier-plugin-astro"],
+  overrides: [
+    {
+      files: "*.astro",
+      options: {
+        parser: "astro",
+      },
+    },
+  ],
+};