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  generation guard keeps a superseded fetch from publishing (speed-test pattern).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/session/src/main.rs

@@ -52,6 +52,15 @@ mod session_main {
             || std::env::var_os("GAMESCOPE_WAYLAND_DISPLAY").is_some()
     }
 
+    /// `--window-pos X,Y` → the window's top-left in desktop coordinates (a spawning
+    /// shell passes its own position so the session opens on the same monitor); absent or
+    /// unparsable = centered on the primary display.
+    pub(crate) fn window_pos() -> Option<(i32, i32)> {
+        let v = arg_value("--window-pos")?;
+        let (x, y) = v.split_once(',')?;
+        Some((x.trim().parse().ok()?, y.trim().parse().ok()?))
+    }
+
     /// `host[:port]`, port defaulting to the native 9777.
     pub(crate) fn parse_host_port(target: &str) -> (String, u16) {
         match target.rsplit_once(':') {
@@ -308,6 +317,7 @@ mod session_main {
         let opts = pf_presenter::SessionOpts {
             window_title: format!("Punktfunk · {title}"),
             fullscreen,
+            window_pos: window_pos(),
             print_stats: settings.show_stats || arg_flag("--stats"),
             json_status: true,
             on_connected: Some(Box::new(|fingerprint: [u8; 32]| {