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/Dockerfile

@@ -13,6 +13,11 @@ ARG NPM_REGISTRY=https://registry.npmmirror.com
 COPY package.json package-lock.json ./
 RUN npm config set registry ${NPM_REGISTRY} && npm ci
 
+# Developer Mode panel on the login page (default enabled; set to
+# "false" to build a production login without the dev panel).
+ARG DEV_LOGIN_ENABLED=true
+ENV VITE_ENABLE_DEV_LOGIN=${DEV_LOGIN_ENABLED}
+
 # Source code + build
 COPY . .
 RUN npm run build