feat(cloud): support China region for token-based login
The /cloud/token endpoint always used the global Bambu API endpoint,
so users with China-region access tokens could not validate their
token. The password login flow already exposes a region selector; this
brings the token flow to parity.
Cloud credentials were stored globally — one Bambu Cloud account per
Bambuddy instance. When auth was enabled, any user logging into Cloud
overwrote everyone else's credentials. Credentials are now stored
per-user: each user gets their own independent Cloud login.
Also fixed cloud data endpoints (settings, fields, preset CRUD)
requiring settings:read/settings:update permissions instead of
cloud:auth — users who had "Cloud Auth" enabled but "Settings"
disabled couldn't load profiles after logging in.
maziggy