interactive: Host/View entry menu, clipboard copy, player picker

Bare `pixelpass` now opens a dialoguer-driven Host/View menu instead of
going straight to host mode. Host path copies the ticket to the system
clipboard via arboard with silent print-only fallback. View path
prompts for the ticket, then after the local listener binds prompts
mpv-vs-VLC and spawns it detached (setsid + null stdio) so the player
survives pixelpass exiting.

Headless invocations (`pixelpass <ticket>`, `pixelpass --repair`)
unchanged. Per spec at ~/Documents/pixelpass-interactive-mode-spec.md.

src/common/process.rs

@@ -0,0 +1,25 @@
+use std::io;
+use std::os::unix::process::CommandExt;
+use std::process::{Command, Stdio};
+
+/// Spawn a child process fully detached from this process group.
+///
+/// The child gets its own session via `setsid(2)` and null stdio, so it
+/// survives the parent exiting and doesn't take a SIGKILL cascade when
+/// pixelpass dies. The `Child` is dropped immediately — `std::process::Child::drop`
+/// does not kill the process on Unix.
+pub fn spawn_detached(prog: &str, args: &[&str]) -> io::Result<()> {
+    unsafe {
+        Command::new(prog)
+            .args(args)
+            .stdin(Stdio::null())
+            .stdout(Stdio::null())
+            .stderr(Stdio::null())
+            .pre_exec(|| {
+                nix::unistd::setsid().ok();
+                Ok(())
+            })
+            .spawn()?;
+    }
+    Ok(())
+}