Cross-compile groundwork: make the tree build for Windows (3.1)

`cargo check --target x86_64-pc-windows-gnu` is now green (only the
upstream egui f32:From<f64> warnings). The Linux-only desktop deps are
target-gated so the rest of the stack — which already cross-compiles —
can build off Linux:

- Cargo.toml: ksni and rfd's xdg-portal backend move to
  [target.'cfg(target_os = "linux")'.dependencies]; non-Linux gets rfd
  with its native Win32/AppKit backend (default features).
- tray.rs: cfg-split. The ksni/SNI implementation stays Linux-only; other
  OSes get a no-op `start() -> None`, i.e. windowed-only (identical to the
  Linux no-SNI-host path). TrayEvent/TrayHandle stay cross-platform.
- plex.rs: add a Windows `symlink_file` arm to make_symlink (also fixes an
  unused-variable warning on non-unix).

The rest was already portable (disk_space/statvfs, mpv IPC UnixStream,
chmod/PermissionsExt guards all cfg(unix)). macOS is Unix so those apply
there unchanged. Still not a *shipped* binary: needs a real per-OS tray
backend and a linking CI matrix. ROADMAP 3.1 + CLAUDE.md updated.

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

src/tray.rs

@@ -21,7 +21,7 @@
 //! the tray entirely optional — if `TrayController::start` fails the app
 //! continues normally.
 
-use std::sync::mpsc::{self, Receiver, Sender};
+use std::sync::mpsc::Receiver;
 
 /// Menu events emitted by the tray. The main app drains these in
 /// [`eframe::App::update`] and translates them into viewport commands or
@@ -49,8 +49,9 @@ pub struct TrayHandle {
 /// by ksni when the SNI host needs a property; menu activations call the
 /// boxed closures we store in `MenuItem::activate`, which forward into
 /// the channel back to the main thread.
+#[cfg(target_os = "linux")]
 struct TrayModel {
-    tx: Sender<TrayEvent>,
+    tx: std::sync::mpsc::Sender<TrayEvent>,
     /// PNG bytes for the icon, decoded once at construction. ksni asks
     /// for raw ARGB so we keep both forms.
     icon_argb: Vec<u8>,
@@ -58,6 +59,7 @@ struct TrayModel {
     icon_h: i32,
 }
 
+#[cfg(target_os = "linux")]
 impl ksni::Tray for TrayModel {
     fn id(&self) -> String {
         "yt-offline".into()
@@ -127,6 +129,7 @@ impl ksni::Tray for TrayModel {
 /// Returns `None` if the runtime / channel can't be set up. A `Some(_)`
 /// return does *not* guarantee the icon is actually visible — that depends
 /// on whether a StatusNotifier host is registered on the user's desktop.
+#[cfg(target_os = "linux")]
 pub fn start(icon_png_bytes: &[u8]) -> Option<TrayHandle> {
     // Decode the PNG eagerly so we fail fast if the embedded asset is bad.
     let img = image::load_from_memory(icon_png_bytes).ok()?;
@@ -141,7 +144,7 @@ pub fn start(icon_png_bytes: &[u8]) -> Option<TrayHandle> {
         argb.push(px[2]);
     }
 
-    let (tx, rx) = mpsc::channel();
+    let (tx, rx) = std::sync::mpsc::channel();
     let model = TrayModel {
         tx,
         icon_argb: argb,
@@ -175,3 +178,13 @@ pub fn start(icon_png_bytes: &[u8]) -> Option<TrayHandle> {
 
     Some(TrayHandle { events: rx })
 }
+
+/// Non-Linux stub: there's no StatusNotifierItem tray backend on Windows or
+/// macOS yet (ksni is Linux/SNI-only), so the tray is simply absent and the
+/// app runs windowed-only. A real implementation would need a per-OS backend
+/// (e.g. `tray-icon`) behind this same signature. Returning `None` makes
+/// `App` behave exactly as it does on Linux when no SNI host is registered.
+#[cfg(not(target_os = "linux"))]
+pub fn start(_icon_png_bytes: &[u8]) -> Option<TrayHandle> {
+    None
+}