fix(chat): keep idle minimap hidden on wide panes

[beruro]

Summary

• remove the responsive flex utility that overrode the minimap idle-state hidden class on wide chat panes
• preserve the existing responsive width/alignment styling and interaction contract
• include the focused UI sweep documenting why semantic peers are intentional

Validation

• pnpm exec tsc --noEmit --pretty false
• eslint src/engines/ChatPanel/ChatHistory/components/ConversationMinimap.tsx
• focused Chat History Vitest suite: 4 passed
• git diff --check

UI audit

1 fix confirmed, 8 semantic peers kept with documented reasons, 0 additional same-class defects, and 0 abstraction candidates.