fix(ui): boost contrast + purge all warm color remnants

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

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

polish(ui): sidebar breathing room, centred view toggle, active repo, diagram readability

Fix expand button scrolling with diagram

Fix mermaid diagram size: set explicit pixel width from max-width value

Fix mermaid diagram compression: render at natural width, scroll horizontally

Fix thought tag leakage and diagram size

feat: Mermaid diagrams, agent fixes, deployment prep