feat: initial project scaffold — server, client, Android APK

- FastAPI server with audio mixer (Windows + Voicemeeter), media tracking,
  notifications (progress + alerts), telemetry, and system tray .exe build
- Test suite covering notifications, audio mixer, media, telemetry, and API
- E-Ink web client (vanilla JS, DOM API, block meters, swipe gestures)
- Android WebView APK for BOOX Go 7 Color Gen II (Android 13, Kotlin)
- Design decision records in .pi/docs/design/
- PyInstaller build script for server .exe

server/tests/test_audio_mixer.py

@@ -0,0 +1,73 @@
+"""Tests for the audio mixer data models and backend interface."""
+
+from __future__ import annotations
+
+import pytest
+
+from ..audio_mixer import (
+    AudioChannel,
+    AudioMixerManager,
+    AudioMixerState,
+)
+
+
+class TestDataModels:
+    def test_audio_channel_defaults(self) -> None:
+        ch = AudioChannel(name="Master")
+        assert ch.name == "Master"
+        assert ch.volume == 0.0
+        assert ch.muted == False
+        assert ch.peak_left == 0.0
+
+    def test_audio_mixer_state_defaults(self) -> None:
+        state = AudioMixerState(backend="windows")
+        assert state.backend == "windows"
+        assert state.master_volume == 0.0
+        assert state.master_muted == False
+        assert state.channels == []
+
+    def test_audio_mixer_state_with_channels(self) -> None:
+        channels = [
+            AudioChannel(name="A1", volume=0.8, muted=False),
+            AudioChannel(name="B1", volume=0.5, muted=True),
+        ]
+        state = AudioMixerState(
+            backend="voicemeeter",
+            master_volume=0.65,
+            master_muted=False,
+            channels=channels,
+        )
+        assert len(state.channels) == 2
+        assert state.channels[0].name == "A1"
+        assert state.channels[1].muted == True
+
+
+class TestAudioMixerManager:
+    def test_init_with_no_backends(self) -> None:
+        """Manager should initialize even with no backends available."""
+        manager = AudioMixerManager()
+        # Should not raise
+        state = manager.get_state()
+        assert isinstance(state, dict)
+        assert "backend" in state
+        assert "master_volume" in state
+        assert "channels" in state
+
+    def test_get_state_returns_dict(self) -> None:
+        manager = AudioMixerManager()
+        state = manager.get_state()
+        assert isinstance(state, dict)
+        assert state["backend"] in ("windows", "voicemeeter")
+        assert isinstance(state["channels"], list)
+
+    @pytest.mark.asyncio
+    async def test_set_volume_no_backends(self) -> None:
+        """set_volume should not raise when no backends are available."""
+        manager = AudioMixerManager()
+        await manager.set_volume("master", 0.5)  # Should not raise
+
+    @pytest.mark.asyncio
+    async def test_set_mute_no_backends(self) -> None:
+        """set_mute should not raise when no backends are available."""
+        manager = AudioMixerManager()
+        await manager.set_mute("master", True)  # Should not raise