feat: add provider setup and readiness

[almogdepaz]

summary

• add safe readiness detection for claude, codex, gemini, cursor, and pi
• expose authenticated provider readiness through the control api and settings ui
• seed fresh setups with shell plus detected providers without overwriting existing settings
• keep shell removable/toggleable while always exposing it as a built-in readiness option that can be re-added
• preserve custom-command validation and conservative unknown auth reporting

verification

• bun run typecheck
• bun test — 1843 pass, 0 fail across 123 files
• desktop provider settings e2e — 1 pass
• server-only local deploy with broker and sessions preserved
• live provider/settings and built-in shell discovery smoke checks

closes #197