add auto-signup login and localized auth UI

- backend: userLogin auto-registers missing users and returns auto_registered
- frontend: add responsive auth page (login + signup flow via userLogin) with light/dark mode and logo
- i18n: wire language switching, add translations for cs/en/es/de
- ui/tooling: add Font Awesome integration

AGENTS.md

@@ -15,14 +15,27 @@ It describes what the project is, what is already implemented, and what still ne
   - `backend/`
   - `frontend/`
 
-## Current State (as of 2026-02-12)
+## Current State (as of 2026-02-13)
 
 - `README.md` already contains product specification in Slovak and English.
 - Backend DB migrations exist in `backend/src/Maintenance.php` up to version `7`.
 - Backend API methods are implemented in `backend/src/API.php`.
-- Frontend is still template-level:
-  - `frontend/src/App.vue` has placeholder content.
-  - `frontend/src/router/index.ts` has empty `routes: []`.
+- Frontend auth page is now implemented:
+  - `frontend/src/App.vue` renders router view.
+  - `frontend/src/router/index.ts` maps `/` to `frontend/src/views/AuthView.vue`.
+  - `AuthView` serves as login + registration entry (single form, email + password).
+  - Successful login stores `token` and `user_email` in `localStorage`.
+- Frontend i18n is wired:
+  - setup in `frontend/src/i18n/index.ts`
+  - locale files in `frontend/src/locales/{sk,cs,en,es,de}.ts`
+  - language switcher updates locale dynamically.
+- Frontend theme system is implemented:
+  - light/dark mode toggle in auth page
+  - design tokens in `frontend/src/assets/css/style.css` (`:root` variables).
+- App logo is served from `frontend/public/Nutrio.png` (copied from `doc/Nutrio.png`).
+- Font Awesome is installed and registered globally in `frontend/src/main.ts`.
+- `frontend/src/BackendAPI.js` is generated via `backend/scripts/buildTypeScript.php` and should not be edited manually.
+- `frontend/src/BackendAPI.d.ts` provides TS declarations for generated `BackendAPI.js`.
 - `backend/data.json` contains sample meal data (not currently wired into DB/API flow).
 
 ## Backend Architecture
@@ -111,6 +124,11 @@ All actions are invoked through `backend/public/API.php` with `?action=<method_n
   - ingredients can be user-owned or global (`user_id = null`) for read/select
 - Registration/login generate and store user token in DB.
 - `userLogout(token)` invalidates session by setting `token` and `token_expires` to `NULL`.
+- `userLogin(email, password)` now auto-registers user when:
+  - email is valid
+  - user does not exist
+  - then proceeds with normal login/token generation flow
+- `userLogin` response now includes `auto_registered` flag (`true|false`).
 
 ## Known Pitfalls and Notes
 
@@ -122,6 +140,8 @@ All actions are invoked through `backend/public/API.php` with `?action=<method_n
 - Basic token auth is implemented, but token is still passed as plain API parameter.
 - For `array` parameters (for example `ordered_item_ids`), APIlite expects JSON in request payload.
 - APIlite wraps responses with a nested `data` object. Keep this in mind on frontend parsing.
+- `frontend/src/BackendAPI.js` is generated output; regenerate when backend API changes, do not patch manually.
+- In vue-i18n locale strings, `@` must be escaped as `{'@'}` to avoid "Invalid linked format" errors.
 
 ## Local Runbook
 
@@ -132,6 +152,8 @@ Backend:
 - configure DB via `backend/config/*.php` override
 - serve `backend/public` through web server/PHP runtime
 - first API hit triggers `Maintenance->database()` migration flow
+- when backend API signature changes, regenerate frontend API client:
+  - `php scripts/buildTypeScript.php`
 
 Frontend:
 
@@ -143,7 +165,7 @@ Frontend:
 ## Product Behavior Target (what to build next)
 
 - Harden auth (token transport/header strategy, token revoke strategy, brute-force/rate-limits).
-- Build frontend screens for ingredients, meals, meal item editor, diary day, diary range.
+- Build remaining frontend screens for ingredients, meals, meal item editor, diary day, diary range.
 - Connect frontend to implemented backend actions.
 - Add API tests for validation, ownership checks, and totals calculation consistency.
 - Add pagination/filter strategy where list endpoints grow.