Update oauth endpoints to use instance base url

Editor assessment

The subject states the change clearly: OAuth endpoints now use the instance base URL. A reader understands both the mechanism and its purpose, supporting instances served from sub-paths or custom domains, without the diff. The fix type and core scope are accurate. No body appears, and one line on which deployments broke beforehand would have added value, but the subject carries the essential fact for a small, well-bounded correction. No PR reference is present, which is acceptable. Style is clean: imperative mood, no trailing period, moderate length. A clear, honest small fix.