🐛 Fixed theme editor launch gate for allowlisted themes on limited plans (#27970)

- the server's `POST /themes/upload/` blocks every save by a limited
customer using a `.` sentinel that's never in the allowlist, so editing
any theme on a plan with a `customThemes` limit enabled is rejected
regardless of which theme is active
- the launch gate previously checked the active theme name against the
allowlist, so customers whose current theme happened to be allowlisted
could enter the editor only to hit a generic "something went wrong"
toast on save
- the gate at all three editor entry points (Change theme menu,
installed themes list, `/theme/edit/:name` deep link) now uses the same
`.` sentinel as the upload endpoint, so the LimitModal fires at launch
with the host's customThemes error message instead