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  generation guard keeps a superseded fetch from publishing (speed-test pattern).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

@@ -88,6 +88,23 @@ pub(crate) fn page_wide(children: Vec<Element>) -> Element {
     scroll_view(col).into()
 }
 
+/// Lay tiles into a `cols`-wide grid of equal-width star columns (rows share the height of
+/// their tallest tile, so a grid row always lines up). Shared by the hosts page's host
+/// tiles and the library page's posters.
+pub(crate) fn tile_grid(tiles: Vec<Element>, cols: usize, gap: f64) -> Element {
+    let rows = tiles.len().div_ceil(cols);
+    let mut children = Vec::with_capacity(tiles.len());
+    for (i, t) in tiles.into_iter().enumerate() {
+        children.push(t.grid_row((i / cols) as i32).grid_column((i % cols) as i32));
+    }
+    grid(children)
+        .columns(vec![GridLength::Star(1.0); cols])
+        .rows(vec![GridLength::Auto; rows])
+        .column_spacing(gap)
+        .row_spacing(gap)
+        .into()
+}
+
 /// A page header: a large bold title on the left, one action button on the right.
 pub(crate) fn page_header(title: &str, action: Button) -> Element {
     grid((