Refactor var to const/let in admin/system + log search multi-term (4/4)

[limad]

Summary

Fourth of four migration PRs splitting #3297 by functional domain. This batch covers 12 admin/system files plus the multi-term log search refactor from #3270:

user.class.js     plugin.class.js   log.class.js
config.class.js   cron.class.js     backup.class.js
update.class.js   recovery.class.js report.class.js
note.class.js     interact.class.js appMobile.class.js
desktop/js/log.js  (incorporates #3270)

Log search multi-term (#3270)

desktop/js/log.js adopts the multi-term search refactor proposed in #3270, but with const/let rather than var. The search input now accepts comma-separated terms with per-term :not() exclusion:

• error — single term (unchanged behavior)
• error,warning — match any of the terms
• :not(debug) — exclude single term
• error,:not(daemon) — match error OR exclude daemon

No incidental fixes in this batch

All files in this batch follow the uniform AJAX boilerplate pattern, so the migration is purely mechanical (var → const for never-reassigned, let for the rest).

Test plan

• Open the Logs page — verify the log list and selection works
• Search a log file using the new multi-term syntax (error,warning, :not(debug)) — verify highlighting
• Open Administration → Backup, Cron, Update, Health — verify each tab loads
• Open the User management page — verify user list and edit modal
• Open the Interactions page — type a query — verify response
• Open the Plugin marketplace — install/update flow remains functional
• Verify in mobile mode (?v=m) that the dashboard still renders (appMobile.class.js)

Part of the split following #3297 discussion. After this PR lands, all core/js/*.js files use const/let exclusively, and the ESLint workflow from #3299 can be tightened to enforce no-var and prefer-const permanently.