Set My Theme

Self-service default-theme update for the current user.

Same posture as /me/language: NOT gated by a module permission, because every user (including floor workers whose role cannot read or write personnel records) must be able to choose how their own screen looks.

Writes users.preferred_theme ONLY. It is deliberately not mirrored to the personnel record the way language is: language drives SMS and notification templates that reach the worker off-device and that a manager legitimately sets on the HR record, whereas a theme affects nothing outside this person's browser.

This sets the account DEFAULT, which is the middle rung of the cascade in frontend/src/theme/theme-provider.js. The sidebar switcher does not call this - it writes a device-local override and leaves this column alone.