feat(clients/windows): game library page + session window placement + help polish

UX polish batch 2 (plan workstreams D/E5/E6):

- D: a mouse/keyboard game library page over pf-client-core::library (the GTK
  ui_library.rs counterpart): responsive 2-6 column poster grid (2:3 portraits,
  store badge, monogram placeholder while art streams in), loading / error+retry
  / empty / grid states, tap-to-play via a normal spawn carrying --launch id.
  Poster bytes land in a disk cache (%LOCALAPPDATA%\punktfunk\art-cache) so the
  Image widget loads file:/// URIs (reactor has no from-bytes source) and
  revisits render instantly. Reached from a paired host's "Browse library..."
  menu item behind the new "Show game library (experimental)" Settings toggle
  (the shared library_enabled field, Apple/GTK parity). A Shared::library_gen
  generation guard keeps a superseded fetch from publishing (speed-test pattern).

- E5: the session window opens at the shell's own top-left (--window-pos, new
  SessionOpts::window_pos) instead of centered on the primary display - streams
  land on the monitor the shell is on, and the hide/restore handoff reads as one
  window changing content. Fullscreen follows that display.

- E6: the Help shortcuts add Alt+Enter and the controller escape chord
  (LB+RB+Start+Back, hold to disconnect).

Also: rustfmt normalization of the merged probe helper.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/windows/src/spawn.rs

@@ -97,6 +97,7 @@ pub(crate) fn session_binary() -> std::path::PathBuf {
 /// streams fullscreen" toggle); `launch` carries a library title id for the host to
 /// launch during the handshake. `Err` = the spawn itself failed (binary missing?) —
 /// surfaced as a connect error by the caller.
+#[allow(clippy::too_many_arguments)] // one cohesive spawn spec (session_params precedent)
 pub(crate) fn spawn_session(
     addr: &str,
     port: u16,
@@ -120,6 +121,7 @@ pub(crate) fn spawn_session(
     if let Some(id) = launch {
         cmd.arg("--launch").arg(id);
     }
+    add_window_pos(&mut cmd);
     spawn_with(cmd, &format!("{addr}:{port}"), slot, on_event)
 }
 
@@ -141,9 +143,19 @@ pub(crate) fn spawn_browse(
     if fullscreen {
         cmd.arg("--fullscreen");
     }
+    add_window_pos(&mut cmd);
     spawn_with(cmd, &format!("{addr}:{port}"), slot, on_event)
 }
 
+/// Hand the shell window's position to the child (`--window-pos`) so the session window
+/// opens on the same monitor, where the shell is — the hide/restore handoff then reads as
+/// one window changing content instead of a window jumping displays.
+fn add_window_pos(cmd: &mut Command) {
+    if let Some((x, y)) = crate::shell_window::position() {
+        cmd.arg("--window-pos").arg(format!("{x},{y}"));
+    }
+}
+
 /// The shared spawn + stdout-contract reader behind [`spawn_session`]/[`spawn_browse`].
 fn spawn_with(
     mut cmd: Command,