docs: Add README

https://claude.ai/code/session_01ToKgc86ZWSEyGxQBcaridp

Author: claude

README.md

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+# Whisper Transcriber
+
+Android floating overlay app for voice-to-text using a self-hosted [whisper.cpp](https://github.com/ggml-org/whisper.cpp) server. Tap the bubble, speak, and the transcription is typed directly into whatever text field you're using.
+
+Works over Tailscale / ZeroTier — just point it at your server's VPN IP.
+
+## How it works
+
+1. A floating bubble sits over all apps (like Messenger chat heads)
+2. Tap to start recording, tap again to stop
+3. Audio is sent to your whisper.cpp server via the OpenAI-compatible API (`/v1/audio/transcriptions`)
+4. Transcribed text is automatically typed into the focused input field (or copied to clipboard)
+
+## Setup
+
+### Server
+
+Run [whisper.cpp server](https://github.com/ggml-org/whisper.cpp) on your machine:
+
+```bash
+./whisper-server -m models/ggml-base.en.bin --port 8080
+```
+
+### App
+
+1. Install the APK (grab from [Actions artifacts](../../actions) or build yourself)
+2. Open the app, go to **Settings**, enter your server URL (e.g. `http://10.147.20.13:8080`)
+3. Grant permissions when prompted:
+   - **Microphone** — for recording audio
+   - **Display over other apps** — for the floating bubble
+   - **Notifications** — for the foreground service
+4. Enable the **Whisper Transcriber** accessibility service in Android Settings → Accessibility (needed to type into other apps' text fields)
+5. Tap **Start Overlay** — the floating bubble appears
+
+### Permissions
+
+| Permission | Why |
+|---|---|
+| `RECORD_AUDIO` | Capture voice from microphone |
+| `SYSTEM_ALERT_WINDOW` | Floating bubble overlay |
+| `FOREGROUND_SERVICE` | Keep the overlay alive |
+| `INTERNET` | Send audio to whisper server |
+| `POST_NOTIFICATIONS` | Foreground service notification (Android 13+) |
+| Accessibility Service | Type transcription into focused text fields |
+
+## Building
+
+### With Nix (CI uses this)
+
+```bash
+nix develop --command ./gradlew assembleDebug
+```
+
+The `flake.nix` provides JDK 17 + Android SDK (platform 34, build-tools 34.0.0).
+
+### Without Nix
+
+Requires JDK 17 and Android SDK with platform 34:
+
+```bash
+export ANDROID_HOME=/path/to/android/sdk
+./gradlew assembleDebug
+```
+
+APKs end up in `app/build/outputs/apk/`.
+
+## CI
+
+GitHub Actions builds debug + release APKs on every push using a self-hosted NixOS runner. Artifacts are retained for 7 days, older ones are cleaned up automatically.
+
+## Project structure
+
+```
+app/src/main/java/com/whispertranscriber/
+├── MainActivity.kt              # Home screen, nav, permissions
+├── audio/
+│   └── AudioRecorder.kt         # Mic recording → WAV conversion
+├── data/
+│   ├── SettingsStore.kt          # DataStore-backed preferences
+│   └── TranscriptionLog.kt      # Transcription history (last 100)
+├── network/
+│   └── WhisperApiClient.kt      # OkHttp multipart POST to whisper server
+├── service/
+│   ├── FloatingOverlayService.kt          # Bubble UI + record/transcribe flow
+│   └── TranscriberAccessibilityService.kt # Types text into focused fields
+└── ui/
+    ├── LogScreen.kt              # Transcription history viewer
+    ├── SettingsScreen.kt         # Server URL + audio quality config
+    └── theme/Theme.kt            # Material 3 theme
+```
+
+### whisper-client (Rust crate)
+
+`whisper-client/` contains an async Rust library for calling a Whisper API with either API key auth or [Cashu](https://cashu.space) ecash payment (using [cdk](https://github.com/cashubtc/cdk) 0.8). This is a standalone library, not used by the Android app.
+
+```rust
+let client = WhisperClient::new("https://whisper.example.com".into());
+
+// With API key
+let result = client.transcribe_with_key(
+    "sk-...", audio_bytes, "recording.wav", TranscribeOptions::default()
+).await?;
+
+// With Cashu payment (10 sats/minute)
+let result = client.transcribe_with_cashu(
+    &wallet, 10, audio_bytes, "recording.wav", TranscribeOptions::default()
+).await?;
+
+println!("{}", result.text);
+```
+
+## Network notes
+
+- **HTTP** works out of the box to any IP (cleartext traffic is allowed via network security config)
+- **HTTPS with self-signed certs** works — the client trusts all certificates (this is a private VPN tool, not a public app)
+- Works over **Tailscale**, **ZeroTier**, or any VPN — just use the VPN IP as the server URL
+
+## Tech stack
+
+- Kotlin + Jetpack Compose + Material 3
+- OkHttp for network
+- DataStore for preferences
+- Target SDK 34, min SDK 26
+- Gradle 8.5, AGP 8.2.2