feat: CSRF protection on the login and URL-generator forms (closes #93)

Both cookie-authenticated HTML form posts (POST / and POST /generate)
now require a CSRF token via gorilla/csrf, the recorded default in
GO_PACKAGE_DEFAULTS.md. The token cookie is independent of the session
cookie, so it also covers the login POST, where no session exists yet
(login CSRF). The token key is derived from the signing key with its own
HKDF salt, so tokens survive restarts and reuse no other key material;
gorilla/csrf supplies crypto/rand generation and constant-time compare.

The form routes sit in a chi group behind the middleware; the hidden
token field is rendered into login.html and generator.html. In local
plaintext HTTP mode (debug) requests are marked plaintext so the library
does not demand an https Referer or set a Secure cookie the browser
would withhold; in production, behind the TLS-terminating proxy, it
enforces its https Referer origin check.

Model: opus-4-8

internal/templates/generator.html

@@ -47,6 +47,7 @@
         {{end}}
 
         <form method="POST" action="/generate" class="bg-white rounded-lg shadow-md p-6 space-y-4">
+            {{ .CSRFField }}
             <div>
                 <label for="url" class="block text-sm font-medium text-gray-700 mb-1">
                     Source URL