[1/6] Refine settings center navigation / 优化设置中心导航与文案

[SivanCola]

Summary / 摘要

• Refine settings center labels, stable tab subtitles, and navigation density.
• Hide redundant current-session model information unless it differs from the default or is unavailable.
• Tone down active navigation styling for a quieter settings surface.

Validation / 验证

• npm run typecheck && npm run build
• go test . -run 'TestPreviewSessionMessages|TestHistoryMessages|TestToWireTool' (desktop)\n- go test ./internal/event ./internal/serve ./internal/agent ./internal/control\n- git diff --check