refactor: replace custom HMAC CSRF with hono/csrf middleware

Removes 38-line hand-rolled HMAC-SHA256 implementation in favour of
the built-in hono/csrf, which validates the Origin header natively.

- Delete src/utils/csrf.ts
- Replace custom CSRF middleware with hono/csrf (Origin-header check)
- Remove csrfToken from ContextVariableMap, layout(), forms, and JS fetch() calls
- Update admin tests: swap X-CSRF-Token for Origin header

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/types/hono.d.ts

@@ -4,6 +4,5 @@ import { Env } from "./index";
 declare module "hono" {
   interface ContextVariableMap {
     env: Env;
-    csrfToken: string;
   }
 }