fix(terminal-types): handle user-pinned Pi sessions and explicit type selection

[nkarpov]

Summary

Fixes two regressions observed when using a custom default terminal type pinned to user auth.

• frontend: always include typeId in POST /api/sessions so selecting built-in terminal works even when another type is the configured default
• shared launcher bootstrap: accept either OAuth client credentials or DATABRICKS_TOKEN
• shared launcher bootstrap: write .databrickscfg using token auth when running in user-token mode
• docs: clarify that shared bootstrap supports OAuth and user-token auth checks

Problem

1. Selecting terminal in the picker still created the default custom type, because the frontend omitted typeId when terminal was selected.
2. Pi-based types pinned to authPolicy=user failed bootstrap with:
   • Missing DATABRICKS_CLIENT_ID
   • Missing DATABRICKS_CLIENT_SECRET

Validation

• npm run check
• npm test
• npm run build