Smarter multilingual language detection and switching

Collaborative language model with sticky explicit requests, a per-turn language directive to the LLM, and a conversation-language state machine with backchannel hysteresis so a stray "yes/हाँ" no longer flips the language. Full language packs for all 13 dashboard languages with variant normalization, plus end-to-end hardening for Bengali, Tamil, Marathi, Telugu, and Gujarati. Transliteration-aware detection handles Soniox romanized output via romanized function-word packs and phonetic cross-script matching. Negation-aware switch requests — "don't speak Hindi" is no longer treated as a request for Hindi.