site: add Astro frontend — login, register, reset-password, invites, home

Step 8 of the migration plan. Minimal Astro + Tailwind site (no Svelte).
Pages:
- / (home): post-login card grid, shows Activity/Wiki/Planner cards based
  on wiki_access / activity_access from /api/me; URLs via PUBLIC_* env vars
- /login/: JWT cookie issued on success; ?next= redirect supported
- /register/: invite-code flow, auto-fills code from ?code= param
- /reset-password/: admin-issued code flow; disables form on success
- /invites/: list + generate invites; activity-access toggle for eligible users

Base layout: minimal nav with handle + sign-out, auth wall (/api/me check),
race-calendar accent palette, dark/light theme tokens.

site/tailwind.config.mjs

@@ -0,0 +1,7 @@
+/** @type {import('tailwindcss').Config} */
+export default {
+  content: ["./src/**/*.{astro,html,js,ts}"],
+  darkMode: "class",
+  theme: { extend: {} },
+  plugins: [],
+};