v1.1.0 - Robust model lifecycle & busy UI

Hardened model download and readiness checks (GGUF magic + size validation), prevented crashes on Ask when model is missing or partial, added responsive busy-state UI for Compare/Infer/Ask, moved diff comparison and workbook writing off the UI thread for large files (verified with 8000-row workbooks). Requires .NET 10 Desktop Runtime (win-x64).