feat: refine markdown and tool activity presentation

[chenyme]

Summary

Refresh markdown rendering, unify reasoning/search/tool activity rows, adopt the shadcn shimmer utility, refine task-list lifecycle behavior, and synchronize desktop and WebUI presentation.

Changes

• refresh markdown typography and add collapsible, copyable code blocks
• render reasoning content through the shared markdown renderer
• unify reasoning, web search, tool calls, and tool groups as lightweight icon-title rows
• add consistent expand and collapse transitions across activity sections
• standardize running and completed status labels across desktop and WebUI
• replace legacy loading animations with the shadcn shimmer utility
• show only the latest task-list snapshot and hide stale TodoWrite results
• keep incomplete task lists expanded and collapse them after completion
• remove the redundant return label from task-list results
• suppress duplicate live tool status while a running tool row is visible
• remove inconsistent cards, borders, dividers, badges, and typing dots
• align assistant loading and streaming states across desktop and WebUI
• polish the sidebar update button and its hover interaction
• add Chinese and English translations for markdown controls and thinking states

Validation

• pnpm lint — desktop
• pnpm exec tsc --noEmit — desktop
• pnpm lint — WebUI
• pnpm exec tsc --noEmit — WebUI
• node scripts/check-mirror.mjs
• git diff --check

All checks pass. Existing repository lint warnings remain unchanged.