feat: remove login requirements for internal home-network use

- backend/app/security.py: require_session() now auto-authenticates by
  returning the first family_profile_id from the DB. No cookie or password
  needed. Falls back to "bootstrap" sentinel if no FamilyProfile exists.
  Admin routes (require_admin) still protected by bearer token.
- frontend/src/api/index.ts: removed 401→/login redirect interceptor
- frontend/src/App.tsx: removed Sign out button, removed /login route and
  Login page import
- Login page kept on disk (unused) for potential future re-enablement

frontend/postcss.config.js

@@ -0,0 +1,6 @@
+export default {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+}