fix(auth): preserve linked identities across providers

Summary

• Preserve the currently signed-in account when connecting another identity provider.
• Add Accounts.link_identity/2 to attach provider identities without moving identities owned by another user.
• Keep anonymous OAuth callbacks on the existing sign-in path while redirecting linked accounts back to account identities.
• Cover account-linking, idempotency, and already-linked identity handling in tests.

Testing

• mix test test/hive/accounts_test.exs test/hive_web/controllers/auth_controller_test.exs test/hive_web/live/account_live/identities_test.exs
• mix compile --warnings-as-errors

No GitHub comments yet.