feat: update documentation, adjust numbering of arc42 files

frontend/README.md

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+# Frontend — Flask Web UI
+
+Server-rendered UI powered by Flask + Jinja2 + Tailwind CSS.
+
+## Tech Stack
+
+| Layer       | Choice                           |
+| ----------- | -------------------------------- |
+| Framework   | Flask                            |
+| Templates   | Jinja2                           |
+| Styling     | Tailwind CSS (CDN) + custom CSS  |
+| HTTP client | httpx                            |
+| Server      | gunicorn (prod), Flask dev (dev) |
+
+## Project Structure
+
+```txt
+frontend/
+  app/
+    __init__.py          App factory (create_app)
+    main.py              Entry point — calls create_app()
+    config.py            Configuration (secret key, backend URL)
+    filters.py           Jinja2 template filters (fromisoformat, humantime)
+    helpers.py           API client (_api), auth decorators, model helpers
+    routes/              Blueprint route handlers
+      __init__.py        register_blueprints()
+      auth.py            /login, /register, /logout, /
+      dashboard.py       /dashboard
+      gallery.py         /gallery, image/video/upload detail, /images/<id>/file
+      generate.py        /generate/text, /generate/image, /generate/video
+      admin.py           /admin, /admin/models, /api/admin/*
+      profile.py         /users/profile
+    templates/           Jinja2 HTML templates
+      admin/             Admin sub-templates (models, videos)
+    static/              CSS (style.css), JS (app.js)
+  tests/                 pytest suite
+  Dockerfile             Production container
+  requirements.txt       Runtime dependencies
+  requirements-dev.txt   Dev dependencies
+```
+
+## Running locally
+
+```bash
+cd frontend
+flask --app app.main run --port 12016 --debug
+```
+
+## Running tests
+
+```bash
+pytest frontend/tests/
+```
+
+## Architecture
+
+See [docs/ARCHITECTURE.md](../docs/ARCHITECTURE.md) for full documentation (arc42 template).