fix(api): add Content-Type header to POST/PATCH requests

From the PR description

Bug fix: "Valid email is required" when creating users

Root cause: apiRequest() in mikeApi.ts did not set Content-Type: application/json in the request headers. Express express.json() middleware only parses the request body when Content-Type is application/json. Without it, req.body was empty, so email was undefined → backend returned 400 "Valid email is required".

Fix: Add Content-Type: application/json to the default headers in apiRequest().

Affected: All POST/PATCH admin endpoints (create user, change role, disable, enable, reset password, revoke sessions).

Tests: 85 frontend tests green, typecheck clean, lint clean.