Windows compat quick wins: echo-cancel UI gate, pixelpass .exe, cfg tighten (W5/W6/W9)

Three Windows-compatibility fixes from the Codex review. Implemented by Codex
(gpt-5.5); reviewed and committed by Claude.

W5 — echo cancellation is a Linux/PipeWire feature, but the toggle was shown and
live on Windows, so a Windows join tried `pactl` and errored before falling back.
Now `#[cfg(target_os = "linux")]` gates the core enable path (and the
ActiveSession guard field); on other targets the Settings + in-call controls
render as a disabled checkbox with a "not available on Windows yet" note.

W6 — pixelpass PATH lookup only tried `pixelpass`; on Windows it now also tries
`pixelpass.exe` via a cfg-selected candidate list (+ unit test).

W9 — the Linux audio stack (pipewire/pw_cli/echo_cancel/audio_probe + the
`PlatformAudioBackend` alias and device-enum re-export) was gated `cfg(unix)`;
tightened to `cfg(target_os = "linux")` so a hypothetical macOS build won't try
to compile PipeWire. cpal stays `cfg(windows)`. Genuinely-Unix file/key
permission code in lib.rs/identity.rs left as `cfg(unix)`.

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

src/audio/mod.rs

@@ -56,17 +56,18 @@ pub trait AudioBackend: Send + Sync {
     fn stop(&self) -> Result<(), AudioError>;
 }
 
-pub mod echo_cancel;
 pub mod eq;
 pub mod gate;
 pub mod limiter;
 pub mod multitrack;
 pub mod pan;
-#[cfg(unix)]
+#[cfg(target_os = "linux")]
+pub mod echo_cancel;
+#[cfg(target_os = "linux")]
 pub mod pipewire_impl;
 #[cfg(windows)]
 pub mod cpal_impl;
-#[cfg(unix)]
+#[cfg(target_os = "linux")]
 pub mod pw_cli;
 pub mod recorder;
 
@@ -90,7 +91,7 @@ impl std::fmt::Display for AudioDevice {
 // Enumerate audio input/output devices for the pickers (sorted by description),
 // returning the same `AudioDevice` shape regardless of platform: PipeWire
 // (`pw-cli`) on Linux, cpal/WASAPI on Windows.
-#[cfg(unix)]
+#[cfg(target_os = "linux")]
 pub use pw_cli::enumerate_audio_devices;
 #[cfg(windows)]
 pub use cpal_impl::enumerate_audio_devices;
@@ -102,9 +103,9 @@ pub use cpal_impl::enumerate_audio_devices;
 /// platform selection lives entirely here. Both implementations satisfy the
 /// [`AudioBackend`] trait, which is the only interface the core talks to.
 ///
-/// - Linux/Unix → PipeWire ([`pipewire_impl::PipeWireBackend`]).
+/// - Linux → PipeWire ([`pipewire_impl::PipeWireBackend`]).
 /// - Windows → cpal/WASAPI ([`cpal_impl::CpalBackend`]).
-#[cfg(unix)]
+#[cfg(target_os = "linux")]
 pub type PlatformAudioBackend = pipewire_impl::PipeWireBackend;
 #[cfg(windows)]
 pub type PlatformAudioBackend = cpal_impl::CpalBackend;