Move usage commands, provider discovery, fallback scheduling and request lifecycles into UsageController. Keep quota presentation, persistence, account coordination and notifications in the applet, with bounded pure response normalization in UsageResponse.

Resolve review findings on duplicate startup refreshes and integration-test timeout margin; remove obsolete fixture stubs. Update ownership and testing documentation. Existing settings, user-visible behavior, translations and parity TODO remain unchanged.

Validation: make check, packaging, popup smoke, panel matrix and installed Plasma runtime checks passed. PR #256 records exact commits and CI results.