feat: multi-provider LLM support, prominent chat, design pass, and new analytics

feat: Decision Stack redesign - 9 routes collapse into 4 (Verdict / Signals / Financials / Chat)

test: update component assertions for refonte + add 4 new helper tests

fix: add AI badge to earnings_quality_card; update test from deleted quality_signal_chip

feat: mark earnings quality signal rationale as AI interpretation

feat: mark tension card readings as AI interpretation

feat: add ai_badge and interpretation_card helpers