refactor(client): relm4 desktop shell; delete legacy presenter + coverflow (phase 5)

The GTK client becomes a relm4 component tree: AppModel owns the window,
trust gate (rules 1-3), and the spawned session child's lifecycle as
typed messages; the hosts page is a child component with a
FactoryVecDeque of host cards — the HostsCallbacks Rc<dyn Fn> bag, the
busy Cell, and the Rc<RefCell<HostsUi>> cross-page pokes are gone.
Trust/settings/library dialogs stay plain GTK, invoked from update with
a ComponentSender.

Deleted with the in-process presenter: ui_stream.rs, video_gl.rs,
ui_gamepad_library.rs, launch.rs, the khronos-egl dep, and the
PUNKTFUNK_LEGACY_PRESENTER hatch. Every stream (and the console library)
now runs in punktfunk-session; the shell spawns it for card connects and
exec's it for --connect/--browse so the Decky wrapper keeps working. The
request-access flow gains --connect-timeout + a CancelHandle that kills
the child. Screenshot scenes re-pointed onto the components (verified:
hosts + library self-capture; the dialog scenes present correctly and
capture under a GL renderer); the gamepad-library scene is gone with the
page.

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

clients/linux/tools/screenshots.sh

@@ -27,7 +27,7 @@ GEOMETRY="${GEOMETRY:-1380x860x24}"
 SETTLE="${SETTLE:-1.2}"
 SHOT_DISPLAY="${SHOT_DISPLAY:-:99}"
 
-if [ "$#" -gt 0 ]; then SCENES=("$@"); else SCENES=(hosts settings trust pair addhost shortcuts library gamepad-library); fi
+if [ "$#" -gt 0 ]; then SCENES=("$@"); else SCENES=(hosts settings trust pair addhost shortcuts library); fi
 
 [ -x "$BIN" ] || {
 	echo "client binary not found: $BIN (build it first: cargo build --release -p punktfunk-client-linux)" >&2