feat(session): headless --pair, so a box with only SSH can enrol

`punktfunk-session --pair <PIN> --connect host[:port]` runs the SPAKE2
ceremony with no window and no toolkit, prints the same
`paired <addr>:<port> fp=<hex>` line as `punktfunk-client --pair`, and
exits. Until now the PIN ceremony lived only in the GTK shell or the Skia
console, so enrolling an embedded/kiosk client meant installing a desktop
on it — or copying the identity store by hand.

Dispatches above every graphics call (the machine may have no display) and
is in the `--no-default-features` build: enrolling must never be the reason
a minimal image pulls in Skia. `--name` defaults to the hostname instead of
the desktop path's hardcoded "Steam Deck".

`forget_placeholder` and `device_name` move into pf_client_core::trust so
the two binaries share one implementation rather than the session
re-deriving them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

clients/session/src/console.rs

@@ -105,7 +105,7 @@ pub fn run(target: Option<&str>) -> u8 {
     };
 
     let opts = ConsoleOptions {
-        device_name: device_name(),
+        device_name: trust::device_name(),
         deck: is_steam_deck(),
     };
     let (overlay, handles) = match SkiaOverlay::console(opts, entry) {
@@ -251,22 +251,6 @@ pub fn run(target: Option<&str>) -> u8 {
     }
 }
 
-/// The machine's name — what the host lists this client as after pairing.
-fn device_name() -> String {
-    #[cfg(target_os = "linux")]
-    if let Ok(s) = std::fs::read_to_string("/etc/hostname") {
-        let s = s.trim();
-        if !s.is_empty() {
-            return s.to_string();
-        }
-    }
-    std::env::var("COMPUTERNAME")
-        .or_else(|_| std::env::var("HOSTNAME"))
-        .ok()
-        .filter(|s| !s.trim().is_empty())
-        .unwrap_or_else(|| "This device".into())
-}
-
 fn host_display_name(name: &str, addr: &str) -> String {
     if name.trim().is_empty() {
         addr.to_string()