Fix admin UI without global jQuery

[moskvin]

Fixes #950 by replacing the admin settings page's global jQuery dependency with vanilla JavaScript. The Search Platform dropdown now loads, preselects the configured platform, and saves changes on Nextcloud 34 where global $/jQuery is no longer available. Also raises the app compatibility max-version to Nextcloud 34.

Related PRs: fulltextsearch_elasticsearch and files_fulltextsearch admin UI fixes should land together.

Related Links

• Fix admin UI without global jQuery fulltextsearch_elasticsearch#473
• Fix files admin settings without global jQuery files_fulltextsearch#365

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[moskvin]

@ArtificialOwl friendly ping - a set of PRs fixing Nextcloud 34 compatibility across the Full Text Search suite:

• Fix admin UI without global jQuery #952 (this PR), files_fulltextsearch#365, fulltextsearch_elasticsearch#473 - fix the admin settings UI, which is broken on NC 34 by the removal of the global jQuery ($).
• Fix Internal Server Error from OC\Server::getL10NFactory() when app navigation is enabled #953 - fixes a boot-time HTTP 500 that occurs when app_navigation is enabled, caused by a static service-container lookup while building the navigation entry (thanks to @AngusMacGyver-DE for helping catch it).

All four are DCO-signed and CI is green. These PRs have been open ~2 weeks; I've been running the fixes in production (currently NC 34.0.1 + Elasticsearch 9.4.3) - indexing and search work and fulltextsearch:test passes end to end. Happy to rebase or adjust.
Together these close #472 and the sibling tracking issues.