design: premium UI polish + agent reliability fixes

feat(ui): Blueprint Dark — complete color system rebuild

feat(ui): cool-dark base theme — warm cream/sienna now pops on Linear-style bg

Fix thought/tool-step arrow alignment

Fix agent thought alignment and chevron consistency

Collapse past agent thoughts once the agent moves to next step

fix: remove redundant mode tag for agent (ToolCallTrace header already shows it)

fix: mode tag correctly shows RAG/Agent, fix streaming ID collision

feat: add RAG/Agent mode tag to chat messages

feat: parent-document retrieval + Gemma 4 as RAG primary

Add KaTeX math rendering via remark-math + rehype-katex

Fix table wrapping: scroll horizontally instead of wrapping cells

Add remark-gfm: render markdown tables properly

feat: Mermaid diagrams, agent fixes, deployment prep

Major feature: interactive diagrams, MCP integration, and bug fixes

checkpoint: pre-UI-overhaul state

Premium redesign: AI-Native Dark Cinema design system — Space Grotesk + violet accent

UI overhaul: design system, animations, accessibility, mobile, source cards

Phase 5: MCP server + Agentic RAG (ReAct loop)

Phase 4: deployment config (Vercel + Render + GitHub Actions CI)

Phase 3: React UI — chat, repo management, streaming, source cards