feat: add local model inference via sd.cpp (Electron-only)
- Scaffold full IPC bridge: preload.js exposes localAI API to renderer
- electron/lib/localInference.js: binary download (Metal-enabled macOS build),
model management, auxiliary file downloads (Qwen3-4B LLM + FLUX VAE for Z-Image),
and generation via sd-cli with DYLD_LIBRARY_PATH + xattr quarantine stripping
- electron/lib/modelCatalog.js: Z-Image Turbo/Base (featured) + SD 1.5/SDXL models
- Z-Image requires 3 components loaded via --diffusion-model + --llm + --vae flags
- ImageStudio: Local/API toggle (Electron-only), local model selector, progress bar
- SettingsModal: tabbed UI with Local Models tab (hidden on web/hosted version)
- LocalModelManager: engine status bar, per-model download cards, auxiliary
component download UI (text encoder + VAE) for Z-Image models
- localInferenceClient.js: safe wrapper with isLocalAIAvailable() guard
- All local features gated behind isLocalAIAvailable() — web version unaffected
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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