fix(host): input follows session per-connect + restore-guard on desktop switch

Two fixes from live Bazzite testing of the managed-Gaming + mid-stream work:

1. Input now FOLLOWS the active session. The host-lifetime injector was pinned to
   the first backend it opened and only reopened on an inject FAILURE — but with
   Feature A keeping the managed gamescope warm, its EIS socket stays alive, so a
   switch to the KDE desktop + reconnect kept injecting into the idle gamescope
   (input silently dead on KDE). injector_service_thread now compares the
   resolved input backend (default_backend() ← PUNKTFUNK_INPUT_BACKEND, set per
   connect by apply_input_env, and on a mid-stream switch) each event and reopens
   when it changes. Fixes input on a Gaming->Desktop reconnect AND Feature B's
   mid-stream input re-route, with no plumbing.

2. Debounced TV-restore no longer yanks you back to gaming. do_restore_tv_session
   now checks detect_active_session(): if a desktop session is active (the user
   switched), it tears down the idle managed gamescope but does NOT restart the
   gaming autologin. Observed live: the restore fired and restarted
   gamescope-session-plus@ogui-steam while the client was already on the KDE
   desktop.

Also: document PUNKTFUNK_SESSION_WATCH (Feature B opt-in) in the Bazzite host.env
and correct the managed-default description. Compiles, clippy/fmt clean, 78 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/vdisplay/gamescope.rs

@@ -266,6 +266,19 @@ fn do_restore_tv_session() {
     }
     stop_session(SESSION_UNIT); // our gamescope/Steam session, so Steam is free for the autologin
     *MANAGED_SESSION.lock().unwrap_or_else(|e| e.into_inner()) = None;
+    // Only bring the gaming autologin BACK if the box is still meant to be in gaming mode. If the
+    // user switched to a desktop session (KDE/GNOME/wlroots) in the meantime, don't yank them back
+    // to gaming — leave the desktop alone. (We still stopped our idle managed session above.)
+    use super::ActiveKind;
+    if matches!(
+        super::detect_active_session().kind,
+        ActiveKind::DesktopKde | ActiveKind::DesktopGnome | ActiveKind::DesktopWlroots
+    ) {
+        tracing::info!(
+            "gamescope: a desktop session is active — not restoring the TV gaming session"
+        );
+        return;
+    }
     for unit in units {
         let _ = Command::new("systemctl")
             .args(["--user", "start", &unit])