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.toml

@@ -32,12 +32,20 @@ tokio-stream = "0.1"
 tower-http = { version = "0.5", features = ["cors", "fs", "compression-gzip"] }
 argon2 = "0.5"
 rand = "0.8"
-# File-picker dialog for the desktop GUI. xdg-portal backend keeps it pure-Rust
-# (zbus, no GTK/libdbus build dep), consistent with notify-rust above.
-rfd = { version = "0.15", default-features = false, features = ["xdg-portal", "tokio"] }
-ksni = { version = "0.3.4", features = ["tokio"] }
 libc = "0.2.186"
 
+# Platform-specific desktop deps. On Linux the tray (ksni — StatusNotifierItem
+# over zbus) and the rfd file picker (xdg-portal backend) stay pure-Rust with no
+# GTK build dep, matching notify-rust. On Windows/macOS there's no SNI tray yet
+# (tray::start is a compiled-out no-op) and rfd falls back to its native backend
+# (Win32 / AppKit) via its default features.
+[target.'cfg(target_os = "linux")'.dependencies]
+ksni = { version = "0.3.4", features = ["tokio"] }
+rfd = { version = "0.15", default-features = false, features = ["xdg-portal", "tokio"] }
+
+[target.'cfg(not(target_os = "linux"))'.dependencies]
+rfd = { version = "0.15", features = ["tokio"] }
+
 [profile.release]
 opt-level = 3
 # Thin LTO does cross-crate inlining without the rust-lld + bundled-sqlite