[positioner] panic on ubuntu/linux when using any of Positioner::Tray*

### env

```
~
❯ uname -a
Linux simpaix 6.11.0-29-generic #29~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC Thu Jun 26 14:16:59 UTC 2 x86_64 x86_64 x86_64 GNU/Linux
```

```toml
// /src-tauri/Cargo.toml

[dependencies]
tauri = { version = "2.3.1", features = ['tray-icon'] }
tauri-plugin-opener = "2"
serde = { version = "1", features = ["derive"] }
serde_json = "1"
tauri-plugin-translator-bindings = { version = "0.1.0", path = "../plugins/tauri-plugin-translator-bindings" }
tokio = { version = "1.43.0", features = ["fs"] }
thiserror = "2.0.12"
tracing-subscriber = {version = "0.3.19", features = ["chrono", "json"] }
tracing = "0.1"
tauri-plugin-clipboard-manager = "2"

[target.'cfg(not(any(target_os = "android", target_os = "ios")))'.dependencies]
tauri-plugin-autostart = "2"
tauri-plugin-global-shortcut = "2"
tauri-plugin-positioner = { version = "2.0.0", features = ["tray-icon"] }
tauri-plugin-updater = "2"
```
### Issue
Panic on:
```rust
if let Some(window) = app.get_webview_window("main") {
window
.as_ref()
.window()
.move_window_constrained(Position::TrayBottomRight); // panics here due to Position::Tray*
window.show();
window.set_focus();
```

```
thread 'main' panicked at /home/z3ntl3/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tauri-plugin-positioner-2.3.0/src/ext.rs:271:17:
Tray position not set
```

If i do something differently than ``Position::Tray*`` , like ``Position::Center``; there is no panic

I also made sure that tray is setup in my app's setup hook before this part get's delegated

#### Additional note
This actually panics on ubuntu and likely on macos too, whereas Windows Positioner::Tray does not panic