fix(host/input): route KWin auto-detect to the fake_input backend

apply_input_env() hard-pinned PUNKTFUNK_INPUT_BACKEND=libei for KWin, and
default_backend() reads that env first — so the auto-detecting host (the
normal `serve` service) ignored the new KwinFakeInput backend and fell back
to the RemoteDesktop portal path that needs a user to approve. Route KWin to
"kwin" (org_kde_kwin_fake_input); GNOME/Mutter stay on libei (no fake_input
there).

Validated live on a Bazzite KDE box via the auto-detect path:
backend=KwinFakeInput, "KWin fake_input ready (no portal)", input events
forwarded with no errors.

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

crates/punktfunk-host/src/vdisplay.rs

@@ -457,7 +457,11 @@ pub fn settle_desktop_portal(_chosen: Compositor) {}
 pub fn apply_input_env(chosen: Compositor) {
     let backend = match chosen {
         Compositor::Gamescope => "gamescope",
-        Compositor::Kwin | Compositor::Mutter => "libei",
+        // KWin: org_kde_kwin_fake_input — direct injection, no RemoteDesktop portal / approval
+        // dialog (headless, the krdpserver path), authorized by the host's shipped .desktop.
+        Compositor::Kwin => "kwin",
+        // GNOME has neither fake_input nor the wlr protocols → RemoteDesktop portal via libei.
+        Compositor::Mutter => "libei",
         Compositor::Wlroots => "wlr",
     };
     std::env::set_var("PUNKTFUNK_INPUT_BACKEND", backend);