feat(windows): surface shortcuts on a Help screen; exit fullscreen when the stream stops

The in-stream keyboard shortcuts were only reachable deep in Settings > Input.
Move them to a dedicated Help screen reached from a Help button on the host
list (mirrors the Licenses screen — a hook-free page routed from root), and drop
the reference card out of Settings so there's one home for it.

Fullscreen (F11) was a window-style toggle that stream stop never undid, so
disconnecting while fullscreen left the GUI borderless-fullscreen. Split
toggle_fullscreen into is_fullscreen/enter_fullscreen/exit_fullscreen sharing a
module-level SAVED_PLACEMENT, and have input::uninstall() call exit_fullscreen()
on teardown. exit_fullscreen is a no-op when nothing was saved, so it's safe on
every stream stop and idempotent with F11 — fullscreen is now streaming-only.

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

clients/windows/src/app/help.rs

@@ -0,0 +1,90 @@
+//! The Help screen: a short note on the in-stream capture model plus a reference of the keyboard
+//! shortcuts — reached from the Help button on the host list. The Windows counterpart of the GTK
+//! client's Keyboard Shortcuts window; the bindings themselves live in [`crate::input`], so both
+//! clients document the same set.
+
+use super::style::*;
+use super::Screen;
+use windows_reactor::*;
+
+/// The in-stream keyboard shortcuts, in the GTK Shortcuts window's order: the chord, then what it
+/// does. Read-only — the bindings themselves live in the input hook ([`crate::input`]).
+const STREAM_SHORTCUTS: &[(&str, &str)] = &[
+    ("F11", "Toggle fullscreen"),
+    (
+        "Ctrl+Alt+Shift+Q",
+        "Release captured input (click the stream to recapture)",
+    ),
+    ("Ctrl+Alt+Shift+D", "Disconnect"),
+    ("Ctrl+Alt+Shift+S", "Toggle the statistics overlay"),
+];
+
+/// A subtle key-cap chip for the shortcuts reference — the chord on a filled, bordered pill.
+fn key_chip(keys: &str) -> Element {
+    border(text_block(keys).font_size(12.0).semibold())
+        .background(ThemeRef::SubtleFill)
+        .border_brush(ThemeRef::CardStroke)
+        .border_thickness(uniform(1.0))
+        .corner_radius(6.0)
+        .padding(edges(8.0, 3.0, 8.0, 3.0))
+        .horizontal_alignment(HorizontalAlignment::Left)
+        .into()
+}
+
+/// A read-only reference card listing the in-stream keyboard shortcuts. One grid, chord chip then
+/// action, so the actions line up across rows.
+fn shortcuts_reference() -> Element {
+    let mut children: Vec<Element> = Vec::new();
+    for (i, (keys, action)) in STREAM_SHORTCUTS.iter().enumerate() {
+        let row = i as i32;
+        children.push(key_chip(keys).grid_row(row).grid_column(0));
+        let action_cell: Element = text_block(*action)
+            .foreground(ThemeRef::SecondaryText)
+            .vertical_alignment(VerticalAlignment::Center)
+            .into();
+        children.push(action_cell.grid_row(row).grid_column(1));
+    }
+    let table = grid(children)
+        .columns([GridLength::Auto, GridLength::Star(1.0)])
+        .rows(vec![GridLength::Auto; STREAM_SHORTCUTS.len()])
+        .column_spacing(12.0)
+        .row_spacing(6.0);
+    card(vstack((
+        text_block("In-stream keyboard shortcuts")
+            .semibold()
+            .margin(edges(0.0, 0.0, 0.0, 8.0)),
+        table,
+    )))
+    .into()
+}
+
+/// The Help screen: a `page`-column with a Back button to the host list, an intro card on the
+/// capture model, and the shortcuts reference. Hook-free — called inline from `root` like the
+/// other static screens.
+pub(crate) fn help_page(set_screen: &AsyncSetState<Screen>) -> Element {
+    let back_btn = button("Back").accent().icon(Symbol::Back).on_click({
+        let ss = set_screen.clone();
+        move || ss.call(Screen::Hosts)
+    });
+
+    let intro = card(
+        vstack((
+            text_block("During a stream").font_size(15.0).semibold(),
+            text_block(
+                "Click the stream to capture your mouse and keyboard \u{2014} the shortcuts below \
+                 then work while you play. Release capture to hand the cursor back to this \
+                 computer, and click the stream again to retake it.",
+            )
+            .font_size(12.0)
+            .wrap()
+            .foreground(ThemeRef::SecondaryText),
+        ))
+        .spacing(8.0),
+    );
+
+    page(vec![
+        page_header("Help", back_btn),
+        intro.into(),
+        shortcuts_reference(),
+    ])
+}