feat(iso): loading screen while bee-web starts

Replace 15s blocking wait with instant Chromium launch showing a
dark loading page that polls /healthz every 500ms and auto-redirects
to the app when ready.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/overlay/usr/local/bin/bee-openbox-session

@@ -2,16 +2,6 @@
 # openbox session: launch tint2 taskbar + chromium, then openbox as WM.
 # This file is used as an xinitrc by bee-desktop.
 
-# Wait for bee-web to be accepting connections (up to 15 seconds)
-i=0
-while [ $i -lt 15 ]; do
-    if curl -sf http://localhost/healthz >/dev/null 2>&1; then
-        break
-    fi
-    sleep 1
-    i=$((i+1))
-done
-
 # Disable screensaver and DPMS
 xset s off
 xset -dpms
@@ -25,6 +15,6 @@ chromium \
     --disable-session-crashed-bubble \
     --disable-features=TranslateUI \
     --start-fullscreen \
-    http://localhost/ &
+    file:///usr/local/share/bee/loading.html &
 
 exec openbox