paintplus: add ai-stack InvokeAI/ComfyUI as a local backend option

PaintPlus's backend already supported invokeai/comfyui providers (generic
"self-hosted, on another machine" remote APIs) but the installer never
exposed them and the compose file never passed the URLs through. Add a
3rd provider choice — shown only when the ai-stack service is installed —
that sets AI_PROVIDER + INVOKEAI_URL/COMFYUI_URL and joins ai-stack's
Docker network (ai-stack_default) so PaintPlus can reach those containers
by name. No cloud key, no extra GPU download: it rides on ai-stack's
already-running InvokeAI/ComfyUI.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nb2vJ8W7bHKx1JXVvpCraH

paintplus/docker-compose.yml

@@ -16,6 +16,10 @@ services:
       - OPENAI_API_KEY=${OPENAI_API_KEY:-}
       - STABILITY_API_KEY=${STABILITY_API_KEY:-}
       - REPLICATE_API_KEY=${REPLICATE_API_KEY:-}
+      - INVOKEAI_URL=${INVOKEAI_URL:-}
+      - INVOKEAI_DEFAULT_MODEL=${INVOKEAI_DEFAULT_MODEL:-flux-dev}
+      - COMFYUI_URL=${COMFYUI_URL:-}
+      - COMFYUI_DEFAULT_MODEL=${COMFYUI_DEFAULT_MODEL:-v1-5-pruned-emaonly.ckpt}
       - CORS_ORIGINS=*
     # DNS servers for reliable external API access (Replicate, etc.)
     dns: