Add a panel-only provider selection with automatic and explicit-empty modes, canonical provider IDs, a four-provider limit, and synthetic previews. Keep the popup, fetching, and notifications on the complete roster.

Share bounded provider roster parsing and process lifecycle across settings pages. Cover stale replies, cancellation, aliases, and empty selections with QtTests.

Update usage/development guides, translations, and Unreleased. TODO is unchanged because the feature reuses the established official Linux CLI contract.

Verified ef40428: full local pinned-image make check and GitHub CI passed, including packaging, graphical smoke, CodeQL, and the OpenGL panel matrix. All review threads are resolved.