feat(linux): game library browser; split app.rs into cli/launch/ui_trust

- library.rs + ui_library.rs: the host's unified game library over the
  management API (the Apple LibraryClient/LibraryView ported) — mTLS with the
  paired identity, host verified by its pinned cert fingerprint (ureq + rustls,
  unified with the workspace rustls 0.23); posters load async with monogram
  placeholders, and picking a title starts a session that asks the host to
  launch it (the library id rides the Hello).
- app.rs (~800 lines lighter) splits into cli.rs (argv/headless
  pairing/--connect/screenshot scenes), launch.rs (mode resolve + session
  worker + event stream into the UI) and ui_trust.rs (TOFU / SPAKE2 PIN /
  delegated-approval dialogs); ui_hosts/ui_stream reworked around the split.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/linux/Cargo.toml

@@ -33,6 +33,11 @@ pipewire = "0.9"
 sdl3 = { version = "0.18", features = ["hidapi"] }
 
 mdns-sd = "0.20"
+# Game-library fetch from the host's management API over mTLS + fingerprint pinning.
+# `ureq` is small + sync (the host uses it too) and its rustls unifies with the
+# workspace's (quinn's) 0.23; the pinning verifier mirrors core's private `PinVerify`.
+ureq = "2"
+rustls = { version = "0.23", features = ["ring"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 anyhow = "1"