Add cloud sprite providers and browser demos

.env.example

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+# Copy this file to .env and set real values there. Never commit .env.
+
+# Required only for cloud generation through provider="polza".
+POLZA_API_KEY=
+
+# Optional default cloud image model. It can be overridden per MCP tool call.
+POLZA_IMAGE_MODEL=openai/gpt-image-1.5
+
+# Required only for native pixel-art generation through provider="pixellab".
+PIXELLAB_API_KEY=
+
+# Optional local paths for the local Diffusers provider.
+# IMAGEGEN_MODEL_DIR=/path/to/sdxl-base
+# IMAGEGEN_LORA_DIR=/path/to/pixel-art-xl
+# IMAGEGEN_LCM_LORA_DIR=/path/to/lcm-lora-sdxl
+# IMAGEGEN_OUTPUT_DIR=/path/to/output
+# IMAGEGEN_DB_PATH=/path/to/feedback.db