feat(frontend): in-page Casdoor login form + env-gated Developer Mode panel

- redesigned /login (Tailwind, RTL): centered username/password/organization
  form posting to POST /api/v1/auth/login — session starts without leaving
  the page; PM lands on the inventory panel, others on the procurement office
- Developer Mode button fixed top-left opens a role/project test panel
  (admin role added) gated by VITE_ENABLE_DEV_LOGIN (DEV_LOGIN_ENABLED build
  arg in Docker, default enabled)
- removed the old Casdoor redirect button, the PKCE ?code= callback, and the
  public casdoor-config.js — the client secret is no longer shipped to the
  browser; backend mediates the password grant instead

frontend/.env.example

@@ -3,3 +3,8 @@
 # For local dev against a remote backend, set e.g.:
 # VITE_API_BASE_URL=http://localhost:8000/api/v1
 VITE_API_BASE_URL=
+
+# Show the "Developer Mode" panel on the login page.
+# true (default) / false — in Docker builds this is controlled by the
+# DEV_LOGIN_ENABLED build arg (see docker-compose.yml), not this file.
+VITE_ENABLE_DEV_LOGIN=true