feat(debug): unified provider/usage debug CLI and GUI tab (cherry-pick PR #74)

Cherry-pick of 238283f + 6a17da2 + c1cc006 + 4679291 from feat/debug-provider-gui.
All 4 review feedback items addressed by contributor:
- tsc blocker: stale re-exports removed
- Log polling: monotonic seq cursor replaces Date.now()
- HTTP tests: /api/debug endpoints covered (auth, CORS, validation, redaction)
- Usage logs: both streams now route through running proxy API

Co-authored-by: Wibias <37517432+Wibias@users.noreply.github.com>