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/hosts.rs

@@ -277,6 +277,10 @@ pub(crate) fn hosts_page(props: &HostsProps, cx: &mut RenderCx) -> Element {
                     let sa = set_show_add.clone();
                     move || sa.call(true)
                 }),
+                button("Help").icon(Symbol::Help).on_click({
+                    let ss = set_screen.clone();
+                    move || ss.call(Screen::Help)
+                }),
                 button("Settings").icon(Symbol::Setting).on_click({
                     let ss = set_screen.clone();
                     move || ss.call(Screen::Settings)