feat: add admin settings and OCC configuration

[vitormattos]

Summary

• add a Nextcloud administration section for Usage Statistics
• add a Vue settings page using @nextcloud/vue
• configure report retention through the existing administration API
• add OCC commands to read and update retention
• cover OCC behavior with Behat
• cover the Vue settings page with Vitest and Playwright
• add Vite, Vue type checking and Stylelint support
• add Nextcloud-style Node, Stylelint and Playwright workflows adapted for this repository
• simplify the README and remove the redundant support/license footer

Commands

occ usage-statistics-server:retention:get
occ usage-statistics-server:retention:set <days>

The retention constraints remain 45 to 3650 days, with 1095 days as the default.