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/astro.config.mjs

@@ -0,0 +1,20 @@
+import { defineConfig } from "astro/config";
+import tailwind from "@astrojs/tailwind";
+
+const apiPort = process.env.VITE_API_PORT || '4040';
+
+export default defineConfig({
+  integrations: [tailwind()],
+  devToolbar: { enabled: false },
+  output: "static",
+  vite: {
+    server: {
+      proxy: {
+        '/api': {
+          target: `http://localhost:${apiPort}`,
+          changeOrigin: true,
+        },
+      },
+    },
+  },
+});