fix(auth): migrate sign-in from legacy /user/* to V3 action endpoints

Legacy GET /user/authenticate and GET /user/lookup_email were returning 404
because the backend has removed those routes. Updated all 5 auth functions in
ae_core__user.ts to use V3 equivalents:

- auth_ae_obj__username_password: GET /user/authenticate → POST /v3/action/user/authenticate (body)
- auth_ae_obj__user_id_user_auth_key: GET /user/authenticate → POST /v3/action/user/authenticate (body)
- send_email_auth_ae_obj__user_id: GET /user/{id}/email_auth_key_url → GET /v3/action/user/{id}/email_auth_key_url
- qry_ae_obj_li__user_email: GET /user/lookup_email → POST /v3/crud/user/search
- auth_ae_obj__user_id_change_password: PATCH /user/{id}/change_password → POST /v3/action/user/{id}/change_password

Credentials are now in the POST body (not query params) for authenticate calls.
Updated two call sites in e_app_sign_in_out.svelte to drop removed null_account_id param.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/app_components/e_app_sign_in_out.svelte

@@ -252,7 +252,6 @@ function handle_lookup_user_email({ email }: { email: string }) {
         .qry_ae_obj_li__user_email({
             api_cfg: $ae_api,
             account_id: $slct.account_id,
-            null_account_id: false,
             email: email,
             log_lvl: 0
         })
@@ -321,7 +320,6 @@ async function handle_change_password() {
             await core_func.qry_ae_obj_li__user_email({
                 api_cfg: $ae_api,
                 account_id: $slct.account_id,
-                null_account_id: false,
                 email: user_email,
                 log_lvl: 0
             });