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  generation guard keeps a superseded fetch from publishing (speed-test pattern).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/app/settings.rs

@@ -343,6 +343,16 @@ pub(crate) fn settings_page(
         let ss = set_screen.clone();
         button("Third-party licenses").on_click(move || ss.call(Screen::Licenses))
     };
+    let library_toggle = setting_toggle(
+        ctx,
+        "Show game library (experimental)",
+        s.library_enabled,
+        |s, on| s.library_enabled = on,
+    )
+    .tooltip(
+        "Adds \u{201C}Browse library\u{2026}\u{201D} to paired hosts \u{2014} pick a game and it \
+         launches in the stream. Mirrors the Apple client's toggle.",
+    );
 
     // The selected section's content — per-control guidance lives on hover tooltips, so the
     // card is just the controls.
@@ -375,7 +385,10 @@ pub(crate) fn settings_page(
             "Audio",
             settings_card(vec![channels_combo.into(), mic_toggle.into()]),
         ),
-        "about" => ("About", settings_card(vec![licenses_button.into()])),
+        "about" => (
+            "About",
+            settings_card(vec![library_toggle.into(), licenses_button.into()]),
+        ),
         _ => (
             "Display",
             settings_card(vec![