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>

.claude/launch.json

@@ -0,0 +1,11 @@
+{
+  "version": "0.0.1",
+  "configurations": [
+    {
+      "name": "design",
+      "runtimeExecutable": "python",
+      "runtimeArgs": ["-m", "http.server", "8765", "--bind", "127.0.0.1", "--directory", "docs"],
+      "port": 8765
+    }
+  ]
+}