Windows port Phase 2: cpal device enumeration

Give the Windows device pickers a real device list (Phase 0/1 left pw_cli
returning nothing off-Linux) and generalize enumeration into a
platform-neutral interface.

- audio/mod.rs: move AudioDevice here (neutral home), gate pw_cli to
  cfg(unix), and re-export enumerate_audio_devices per-platform (pw_cli on
  unix, cpal_impl on windows). Also drop a now-stale "no-op stub" doc note.
- cpal_impl.rs: add enumerate_audio_devices() — iterate the cpal host's
  input + output devices into AudioDevice (name == description == the cpal
  friendly name, which is what resolve() matches target_node against, so a
  saved selection round-trips), sorted by description.
- pw_cli.rs: use super::AudioDevice instead of a local copy; parsing +
  tests unchanged.
- app/mod.rs: one-line import change; the device-picker logic is untouched.

Verified: shipped Linux state green (build --locked, clippy, 316/316,
pw_cli parse tests 6/6); the cpal enumerator compiles against real cpal via
the Linux/ALSA toggle. Runtime device listing on Windows is pending a real
host (M2/M3). WASAPI names are less stable than PipeWire node names, so a
saved device may not always round-trip (falls back to default).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -2,7 +2,7 @@ use crate::core::{CoreController, messages::{CoreCommand, UiEvent}};
 use crate::network::PeerState;
 use crate::notify::{self, Sound};
 use crate::audio::eq::{EqSettings, EQ_GAIN_DB_MAX, EQ_GAIN_DB_MIN};
-use crate::audio::pw_cli::{AudioDevice, enumerate_audio_devices};
+use crate::audio::{AudioDevice, enumerate_audio_devices};
 use crate::config::{AppConfig, NetworkMode, RecordingMode, RoomLayout};
 use crate::hotkeys::{format_binding, HotkeyAction, HotkeyContext, KeyBinding};
 use crate::presence::PresenceMode;