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>

Cargo.lock

@@ -223,6 +223,8 @@ version = "0.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d2f3f79755c74fd155000314eb349864caa787c6592eace6c6882dad873d9c39"
 dependencies = [
+ "async-fs",
+ "async-net",
  "enumflags2",
  "futures-channel",
  "futures-util",
@@ -328,6 +330,17 @@ dependencies = [
  "pin-project-lite",
 ]
 
+[[package]]
+name = "async-net"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b948000fad4873c1c9339d60f2623323a0cfd3816e5181033c6a5cb68b2accf7"
+dependencies = [
+ "async-io",
+ "blocking",
+ "futures-lite",
+]
+
 [[package]]
 name = "async-process"
 version = "2.5.0"