Add microphone picker and device management features

- Implement `list_input_devices` to retrieve and filter input-capable audio devices.
- Create `MicPickerDialog` for selecting and saving microphone settings.
- Add `set_env_value` function to update .env file with selected microphone index.
- Update UI components to integrate microphone selection functionality.
- Enhance tests for microphone device handling and environment variable management.

tests/test_config.py

@@ -0,0 +1,62 @@
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from bolt_pet import config
+
+
+def test_set_env_value_uncomments_existing_line(tmp_path, monkeypatch):
+    (tmp_path / ".env").write_text("#MIC_DEVICE=\nOTHER=1\n", encoding="utf-8")
+    monkeypatch.setattr(config, "HERE", tmp_path)
+
+    config.set_env_value("MIC_DEVICE", "USB Mic")
+
+    lines = (tmp_path / ".env").read_text(encoding="utf-8").splitlines()
+    assert lines == ["MIC_DEVICE=USB Mic", "OTHER=1"]
+    assert config.os.environ["MIC_DEVICE"] == "USB Mic"
+
+
+def test_set_env_value_overwrites_existing_uncommented_line(tmp_path, monkeypatch):
+    (tmp_path / ".env").write_text("MIC_DEVICE=Old Mic\n", encoding="utf-8")
+    monkeypatch.setattr(config, "HERE", tmp_path)
+
+    config.set_env_value("MIC_DEVICE", "New Mic")
+
+    lines = (tmp_path / ".env").read_text(encoding="utf-8").splitlines()
+    assert lines == ["MIC_DEVICE=New Mic"]
+
+
+def test_set_env_value_appends_when_key_is_missing(tmp_path, monkeypatch):
+    (tmp_path / ".env").write_text("OTHER=1\n", encoding="utf-8")
+    monkeypatch.setattr(config, "HERE", tmp_path)
+
+    config.set_env_value("MIC_DEVICE", "New Mic")
+
+    lines = (tmp_path / ".env").read_text(encoding="utf-8").splitlines()
+    assert lines == ["OTHER=1", "MIC_DEVICE=New Mic"]
+
+
+def test_set_env_value_creates_file_if_missing(tmp_path, monkeypatch):
+    monkeypatch.setattr(config, "HERE", tmp_path)
+
+    config.set_env_value("MIC_DEVICE", "New Mic")
+
+    assert (tmp_path / ".env").read_text(encoding="utf-8") == "MIC_DEVICE=New Mic\n"
+
+
+def test_mic_device_parses_a_numeric_index_as_int():
+    # sounddevice only takes an int index as-is; a string (even "1") goes
+    # through substring matching against every device name instead, which is
+    # not what a saved index means — see the tray's mic picker.
+    assert config._mic_device("1") == 1
+    assert isinstance(config._mic_device("1"), int)
+
+
+def test_mic_device_keeps_a_name_as_a_string():
+    assert config._mic_device("USB Mic") == "USB Mic"
+
+
+def test_mic_device_blank_is_none():
+    assert config._mic_device("") is None
+    assert config._mic_device("   ") is None