feat: add play-external IPC for launching media in an external player

Mirrors stremio-glutin-shell's play-external action (PR Stremio/stremio-glutin-shell#2)
so the Windows shell can hand a stream off to mpv, VLC or PotPlayer
when the user picks an external player in the WebUI.

- Allowlist of Windows player URL schemes: mpv://, vlc://, potplayer://.
  iOS/macOS-only schemes (iina, infuse, outplayer, open-vidhub) have
  no Windows handlers and are intentionally not listed.
- mpv has no Windows URL handler: spawn the binary directly with `--`
  separating options from the stream URL. Tries common install paths
  (Program Files, scoop's %LOCALAPPDATA%\Programs\mpv, %LOCALAPPDATA%\mpv)
  before falling back to PATH lookup.
- Spawned with CREATE_BREAKAWAY_FROM_JOB so the JobObject's
  KILL_ON_JOB_CLOSE doesn't kill mpv when the shell exits.
- vlc:// and potplayer:// are routed through `open::that` to use the
  OS protocol handler the user already has registered.

Test: cargo fmt --all -- --check, cargo clippy --all -- -D warnings.

src/stremio_app/app.rs

@@ -298,6 +298,52 @@ impl MainWindow {
                             }
                         }
                     }
+                    Some("play-external") => {
+                        if let Some(arg) = msg.get_params() {
+                            let arg = arg.as_str().unwrap_or("");
+                            let arg_lc = arg.to_lowercase();
+                            const ALLOWED_SCHEMES: &[&str] = &["mpv://", "vlc://", "potplayer://"];
+                            let allowed = ALLOWED_SCHEMES.iter().any(|s| arg_lc.starts_with(s));
+                            if !arg.is_empty() && allowed {
+                                if let Some(stream_url) =
+                                    arg_lc.starts_with("mpv://").then(|| &arg[6..])
+                                {
+                                    // `--` ends mpv's option parsing; the stream URL can't smuggle flags.
+                                    let mpv_paths: Vec<String> = vec![
+                                        std::env::var("ProgramFiles")
+                                            .ok()
+                                            .map(|v| format!("{v}\\mpv\\mpv.exe")),
+                                        std::env::var("ProgramFiles(x86)")
+                                            .ok()
+                                            .map(|v| format!("{v}\\mpv\\mpv.exe")),
+                                        std::env::var("LOCALAPPDATA")
+                                            .ok()
+                                            .map(|v| format!("{v}\\Programs\\mpv\\mpv.exe")),
+                                        std::env::var("LOCALAPPDATA")
+                                            .ok()
+                                            .map(|v| format!("{v}\\mpv\\mpv.exe")),
+                                        Some("mpv.exe".to_string()),
+                                    ]
+                                    .into_iter()
+                                    .flatten()
+                                    .collect();
+                                    for path in &mpv_paths {
+                                        if Command::new(path)
+                                            .arg("--")
+                                            .arg(stream_url)
+                                            .creation_flags(CREATE_BREAKAWAY_FROM_JOB)
+                                            .spawn()
+                                            .is_ok()
+                                        {
+                                            break;
+                                        }
+                                    }
+                                } else {
+                                    open::that(arg).ok();
+                                }
+                            }
+                        }
+                    }
                     Some("win-focus") => {
                         focus_sender.notice();
                     }