refactor(management): finish monitoring decomposition

[ssfun]

Summary

• reduce MonitoringCenterPage from 3818 to 2117 lines by extracting account quota, account panels, realtime details, settings, and model-price management
• move monitoring styles into feature ownership and split the former 6213-line page stylesheet into seven ordered Sass partials
• add account quota, settings, price-model tests and stronger module/style boundary guards

Validation

• bash scripts/validation/repo.sh
• fresh Management v1.18.5: 108 Bun tests, ESLint, TypeScript, production build, and repeated-application invariance
• fresh Core v7.2.94: guarded preflight, targeted tests, rejected reapplication invariance, and server build