fix: detect Windows CLI shims and surface login

[writingdeveloper]

Summary

• resolve Windows npm CLI shims and stable per-user install paths for Codex and Claude
• launch Codex app-server through the resolved Windows command so logged-in installs are not reported as missing
• add a constrained in-app login terminal for Codex and Claude, plus install guidance for missing CLIs
• recognize fresh provider login roots before the first session directory exists

Verification

• npm test: 81 files passed, 926 tests passed, 1 skipped
• npm run build
• npm run qa: console errors 0, page errors 0
• npm run qa:audit: 0 accessibility violations across audited views