fix language confusion and hallucinations: centralized resolver + deterministic abstention

major quality upgrade: 12 improvements across retrieval, citations, UX

feat: context budget packing, LLM classifier, reranker, multi-query, dual Excel ingestion

fix: improve retrieval depth and answer quality (4 fixes)

fix: reduce top_k to 4 and cap chunk chars to stay under Groq 6000 TPM limit

fix SQL answer not shown; show Excel in documents list

initial