feat(rn): [P1] Discover: live Skills + MCP templates

[liuyidi]

Summary

Replace Discover placeholder with live Skills catalog + MCP template install; unhide Discover tab.

WebUI reference

• webui/src/pages/skills/SkillsPage.tsx
• webui/src/pages/skills/skills-ui.tsx
• webui/src/pages/skills/AddConnectorDialog.tsx
• webui/src/hooks/skills/useSkillsCatalog.ts

API

• /api/webui/skills
• /api/settings/mcp-presets + from-template

RN landing

• Rewrite src/app/(tabs)/discover.tsx
• Remove href: null in src/app/(tabs)/_layout.tsx
• Detail sheet; one-click template add first; custom JSON import later

Acceptance

• Discover tab visible
• Skills list from server
• MCP template install works

Depends

• chore: Settings API thin wrapper