fix: loading screen via Go handler instead of file:// HTML

- bee-web.service: remove After=bee-audit so Go starts immediately
- Go serves loading page from / when audit JSON not yet present;
  JS polls /api/ready (503 until file exists, 200 when ready)
  then redirects to dashboard
- bee-openbox-session: wait for /healthz (Go binds fast <2s),
  open http://localhost/ directly — no file:// cross-origin issues
- Remove loading.html static file

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

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

@@ -8,6 +8,13 @@ xset -dpms
 xset s noblank
 
 tint2 &
+# Wait for bee-web to bind (Go starts fast, usually <2s)
+i=0
+while [ $i -lt 30 ]; do
+    if curl -sf http://localhost/healthz >/dev/null 2>&1; then break; fi
+    sleep 1
+    i=$((i+1))
+done
 chromium \
     --disable-infobars \
     --disable-translate \
@@ -15,6 +22,6 @@ chromium \
     --disable-session-crashed-bubble \
     --disable-features=TranslateUI \
     --start-fullscreen \
-    file:///usr/local/share/bee/loading.html &
+    http://localhost/ &
 
 exec openbox