Panel settings gain a Panel providers section listing the enabled
providers in saved order: the panel text, meters, identity, and tooltip
follow the choice (max four meters), while the popup, fetching, and
notifications keep the full roster. An empty value stays automatic;
__none__ stores an explicit empty selection.

The read-only provider roster loader moves into the shared
controllers/ProviderRosterController used by Popup and Panel, and the
pure PanelProviders.js filter owns selection parsing and toggling.

Validation: make check exit 0 (937 Qt bulk tests, desktop-style
geometry runs, Python tests, hardening, AppStream metadata),
make package, and the plasmawindowed smoke suite (only the
localization-fr/de/es/pt_BR scenarios fail, identically on clean main:
those OS locales are not installed here).