[good first issue] UX polish — friendlier copy for error toasts and empty states

[GoDiao]

Background

Several recently-added error UIs (Mermaid/KaTeX render errors, scheduled-task poll failures, terminal eviction warnings) currently surface technical messages. We'd like to soften the copy and make them actionable.

Tasks

• Collect 5–10 error or empty-state messages that read too technical
• Propose friendlier copy in both zh and en (action-oriented, with a hint at the next step)
• Move them into i18n keys if they aren't already
• Submit a single PR — one commit per message is fine

Examples to look at

• Mermaid / KaTeX render error fallback UI
• Scheduled-task 3-consecutive-failure toast
• Terminal LRU eviction notice
• Provider connectivity test failure
• Empty session list / empty MCP server list

Difficulty / Estimate

⭐ — ~1 hour.

Mentor

Maintainer will help land the wording.