feat(client/linux): CI screenshot capture

Host-free UI screenshots of the GTK4/libadwaita client under a virtual X display
(clients/linux/tools/screenshots.sh) — Xvfb + software GL (llvmpipe) + a root-window
grab, one app launch per scene. PUNKTFUNK_SHOT_SCENE routes build_ui to render one
mock-populated REAL view (hosts grid / settings dialog / TOFU + PIN dialogs) and
print PF_SHOT_READY once it has settled; the saved-hosts grid is driven by a seeded
client-known-hosts.json. NON_UNIQUE in shot mode so back-to-back launches don't
collide. The stream scene is deferred — its page needs a live NativeClient.

Gated to stable release tags in a standalone best-effort workflow that builds the
client in the rust-ci image and captures under Xvfb; PNGs upload as a 30-day
artifact, not committed.

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

.gitignore

@@ -13,6 +13,7 @@ clients/apple/PunktfunkCore.xcframework/
 clients/apple/.swiftpm/
 # Generated App Store screenshots (tools/screenshots.sh output; uploaded as a CI artifact)
 clients/apple/screenshots/
+clients/linux/screenshots/
 # Xcode per-user state
 xcuserdata/