docs: add product idea, design prototypes and development documentation

- IDEA.md: product vision from the user's perspective (input for design)
- Architecture, ADRs, data model, REST API overview
- Auth and roles, library storage (NFS, path regexes, scanner, search),
  playback, recommendation system ("My Wave"), track upload queue
- Web, Android (offline, self-update), i18n, design system
- Testing strategy, Gitea Actions CI/CD, deployment, conventions, roadmap
- Clickable web/Android design prototypes and wave button variants
- .claude/launch.json to serve the prototypes locally

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -1,2 +1,8 @@
 # pipu-music
 
+Self-hosted музыкальный сервис: веб и Android-приложение, библиотека на NFS,
+умная бесконечная лента «Моя волна» с учётом вкуса, звучания треков и настроения.
+
+- Идея продукта: [docs/IDEA.md](docs/IDEA.md)
+- Документация для разработки: [docs/README.md](docs/README.md)
+- План: [docs/roadmap.md](docs/roadmap.md)