Update README for single-app architecture

README.md

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+# What's That Music
+
+A desktop app that plays YouTube Music and serves a now-playing overlay for OBS.
+
+The app is a single Electron window that loads `music.youtube.com`. It also runs a small HTTP/WebSocket server on `localhost:9095` that streams the current track to any connected overlay client. Point OBS (or any browser) at `http://localhost:9095/` to see the overlay.
+
+## Run from source
+
+```
+cd desktop-client
+npm install
+npm start
+```
+
+The app opens. Sign in to YouTube Music. Play a track. Visit `http://localhost:9095/` in another browser to see the overlay.
+
+## Build a Windows zip
+
+```
+cd desktop-client
+npm run build -- --win zip --x64
+```
+
+Output: `desktop-client/dist/WhatsThatMusic-<version>-win.zip`. Unzip on a Windows machine and run `WhatsThatMusic.exe`. The app is unsigned, so SmartScreen will warn on first launch — click "More info → Run anyway."
+
+## OBS setup
+
+1. Add a *Browser Source*.
+2. URL: `http://localhost:9095/`.
+3. Width / height: match your scene (the overlay positions itself absolutely; 1920x1080 is fine).
+4. Make sure the app is running before OBS loads the source.
+
+## Tests
+
+```
+cd desktop-client
+npm test
+```
+
+Runs the `node:test` unit suite (`desktop-client/tests/`).
+
+## Layout
+
+- `desktop-client/` — the Electron app (main entry, server module, preload scraper, tray).
+- `overlay/` — static HTML + JS the bundled server serves at `/`.
+- `docs/superpowers/` — design and plan documents.