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/Cargo.toml

@@ -27,11 +27,8 @@ gtk = { package = "gtk4", version = "0.11", features = ["v4_16"] }
 adw = { package = "libadwaita", version = "0.9", features = ["v1_5"] }
 async-channel = "2"
 
-# The VAAPI GL presenter (video_gl.rs): EGL dmabuf import into a GDK-shared context, dlopened
-# at runtime (`dynamic`) so GPU-less boxes and the software path never touch libEGL.
-khronos-egl = { version = "6", features = ["dynamic"] }
-
 serde_json = "1"
 anyhow = "1"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+relm4 = { version = "0.11", features = ["libadwaita"] }