Add unified backend abstraction layer supporting multiple LLM providers:
Backend (Go):
- New backends package with interface, registry, and adapters
- Ollama adapter wrapping existing functionality
- OpenAI-compatible adapter for llama.cpp and LM Studio
- Unified API routes under /api/v1/ai/*
- SSE to NDJSON streaming conversion for OpenAI backends
- Auto-discovery of backends on default ports
Frontend (Svelte 5):
- New backendsState store for backend management
- Unified LLM client routing through backend API
- AI Providers tab combining Backends and Models sub-tabs
- Backend-aware chat streaming (uses appropriate client)
- Model name display for non-Ollama backends in top nav
- Persist and restore last selected backend
Key features:
- Switch between backends without restart
- Conditional UI based on backend capabilities
- Models tab only visible when Ollama active
- llama.cpp/LM Studio show loaded model name
vikingowl
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