fix(kiosk): build against stock bookworm gtk 4.8.3 (drop v4_12 feature)

Pi OS Bookworm + Debian bookworm both ship libgtk-4 4.8.3. No code in
the kiosk uses 4.12+ APIs (compute_bounds, WidgetPaintable, Picture,
add_tick_callback, Fixed, set_content_fit are all <= 4.8). Swap
gtk4 feature v4_12 → v4_8 and drop the bookworm-backports juggling
in CI.

.github/workflows/release-kiosk.yml

@@ -78,14 +78,10 @@ jobs:
           echo "version=$version" >> "$GITHUB_OUTPUT"
 
       - name: Install GTK/GStreamer/WebKit build deps
-        # gtk4-rs 0.9 + the v4_12 feature need libgtk-4-dev >= 4.12. Plain
+        # gtk4-rs 0.9 with the v4_8 feature builds against bookworm's stock
-        # bookworm ships 4.8.3; backports ships 4.12+. Same arrangement we
+        # libgtk-4-dev 4.8.3 — same version that ships on Pi OS Bookworm.
-        # use on the Pi.
         run: |
-          echo "deb http://deb.debian.org/debian bookworm-backports main" \
+          apt-get install -y --no-install-recommends \
-            > /etc/apt/sources.list.d/bookworm-backports.list
-          apt-get update
-          apt-get install -y --no-install-recommends -t bookworm-backports \
             libgtk-4-dev libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev \
             libwebkitgtk-6.0-dev libssl-dev
 

kiosk/Cargo.toml

@@ -6,8 +6,10 @@ description = "BetterFrame kiosk — multi-camera display with GTK4 + GStreamer"
 license = "AGPL-3.0-only OR Commercial"
 
 [dependencies]
-# GTK4 for windowing/layout
+# GTK4 for windowing/layout. v4_8 = Pi OS Bookworm's libgtk-4 version. We
-gtk4 = { version = "0.9", features = ["v4_12"] }
+# don't use any 4.12+ APIs; bumping the feature gate would force pulling
+# from backports for no functional gain.
+gtk4 = { version = "0.9", features = ["v4_8"] }
 
 # GStreamer for RTSP decode
 gstreamer = "0.23"