fix(clients): GTK + Decky polish batch from live Deck/Windows testing

GTK Linux client:
- hosts/library: clicking a card was dead — the handler was on
  FlowBoxChild::activate (never emitted on click); bridge child-activated
  → child.activate() on the FlowBox (ui_hosts, ui_library).
- stream: the Ctrl+Alt+Shift+D/Q/S chords (and all key forwarding) were
  dropped because the key controller sat on the overlay, which loses focus
  to the header back button after nav.push+fullscreen — move it to the
  window and remove it on teardown.
- video: a mid-session VAAPI decode error rebuilt a software decoder but
  never requested a keyframe, so under the infinite GOP the picture stayed
  gray/frozen forever. Request an IDR on any VAAPI error, keep the hardware
  decoder, and demote to software only after repeated failures.
- stream: fix a per-session Capture↔overlay reference cycle that leaked the
  overlay subtree + the Arc<NativeClient> on every session end — hold the
  overlay weakly.
- stream: accumulate the fractional wheel remainder so precision-scroll
  (Deck trackpad / hi-res wheels) sub-unit deltas aren't dropped.
- gamepad library: keep the launcher smooth on the Deck — freeze the aurora
  and trim the visible card range (fewer 3D offscreen passes) on low-power.
- gamepad: log full pad identity (vid:pid:name:type:virtual) on attach to
  diagnose an empty controller list on the Deck.
- cli: --connect host:<badport> silently did nothing; default to 9777 + warn.
- css: add the missing .pf-neutral pill rule; fix the clipped most-recent
  accent (inset outline instead of a corner-clipped box-shadow bar).

Decky plugin:
- surface the on-screen library browser: label the host-row Games button.
- fix silent pin data-loss — the detached Games modal captured a frozen
  pins array, so pinning a second game clobbered the first; mirror pins in
  a ref and track the modal's pinned ids locally for a live label.
- route pair-required hosts through the pairing modal from the fullscreen
  Stream button (parity with the QAM panel).

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

clients/linux/src/ui_library.rs

@@ -71,6 +71,11 @@ fn build(app: Rc<App>, req: ConnectRequest) -> Rc<State> {
         .row_spacing(18)
         .valign(gtk::Align::Start)
         .build();
+    // Click/keyboard activation fires `child-activated` on the FlowBox, not the child's own
+    // `activate` — bridge it so each poster's connect handler (below) runs on click.
+    flow.connect_child_activated(|_, child| {
+        child.activate();
+    });
     let content = gtk::Box::new(gtk::Orientation::Vertical, 0);
     content.set_margin_top(24);
     content.set_margin_bottom(24);