Adds an import flow that lets users bring an existing Codex auth.json
into OmniRoute without a fresh OAuth login. Both a file-upload tab and
a paste-JSON tab are supported.
- `codexAuthImport.ts`: pure parser + createConnectionFromAuthFile
(conflict detection, overwriteExisting, JWT email/exp extraction)
- `POST /api/providers/codex-auth/import`: Zod-validated endpoint with
audit log (`provider.credentials.imported`)
- `importCodexAuthSchema` in schemas.ts (discriminated union json/text,
256 KB cap on paste source)
- `<ImportCodexAuthModal>` in providers/[id]/page.tsx with upload/paste
tabs, email auto-detection, name/email/overwrite fields
- "Import auth" toolbar button shown only on the Codex provider page
- 29 unit tests (17 parser + 12 schema) — all passing
