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/screenshare/mod.rs

@@ -25,6 +25,16 @@ use tokio::process::{Child, Command};
 /// points elsewhere.
 const PIXELPASS_BIN: &str = "pixelpass";
 
+#[cfg(windows)]
+fn pixelpass_path_candidates(dir: &Path) -> [PathBuf; 2] {
+    [dir.join(PIXELPASS_BIN), dir.join("pixelpass.exe")]
+}
+
+#[cfg(not(windows))]
+fn pixelpass_path_candidates(dir: &Path) -> [PathBuf; 1] {
+    [dir.join(PIXELPASS_BIN)]
+}
+
 /// Pixelpass endpoint tickets are normally ~140 chars. Leave headroom for format
 /// growth, but reject unbounded gossip payloads before the UI offers "Watch".
 const MAX_TICKET_LEN: usize = 512;
@@ -143,7 +153,7 @@ pub fn pixelpass_path(config_override: Option<&str>) -> Option<PathBuf> {
     }
     let path_var = std::env::var_os("PATH")?;
     std::env::split_paths(&path_var)
-        .map(|dir| dir.join(PIXELPASS_BIN))
+        .flat_map(|dir| pixelpass_path_candidates(&dir))
         .find(|c| c.is_file())
 }
 
@@ -513,4 +523,14 @@ mod tests {
         // only assert it doesn't return the empty path as a match.
         assert_ne!(pixelpass_path(Some("   ")).as_deref(), Some(Path::new("")));
     }
+
+    #[test]
+    fn pixelpass_path_candidates_are_platform_specific() {
+        let dir = Path::new("bin");
+        let candidates: Vec<PathBuf> = pixelpass_path_candidates(dir).into_iter().collect();
+        #[cfg(windows)]
+        assert_eq!(candidates, vec![dir.join("pixelpass"), dir.join("pixelpass.exe")]);
+        #[cfg(not(windows))]
+        assert_eq!(candidates, vec![dir.join("pixelpass")]);
+    }
 }