Organize and simplify the WordPress admin settings UI

[CodyReichert]

The SimplyRETS settings page is a long form that makes individual options difficult to find. This change organizes it into eight navigable sections, adds a setting search and an All settings view, and provides a persistent save bar with unsaved-change feedback. The layout adapts to narrow screens, and API credentials are concealed by default with a reveal button.

All existing settings and their names, IDs, values, option registration, sanitizers, nonces, and save endpoint are retained. Sections remain in one form so saving includes edits in hidden sections; invalid fields reveal their section. Without JavaScript, every section and the save button remain available.

Cleanup removes the unused admin-page database global, repeated save buttons and separators, superseded CSS declarations, and the inline custom-form script. One WordPress save control retains the existing submit name and ID. The legacy post-editor filter switch is replaced with a label-to-selector map while preserving its control IDs and update order. Search indexes help text and option names once, excluding stored field values. Admin assets use their modification times to invalidate stale caches.

Validation:

• PHP syntax checks on both changed PHP files, JavaScript syntax check, and git diff whitespace check passed.
• Local WordPress/OceanWP browser checks covered desktop and mobile layout, all existing form controls, navigation, search and empty results, custom-form toggling, hidden-field validation, and saving edits across sections. Temporary save-test settings were restored.
• After cleanup, rechecked search/highlighting, custom-form enabled state, unsaved-change feedback, the single save control, and the approved layout.
• Isolated handler comparison passed for all nine legacy filter choices, an unknown choice, and filter removal. Live block-editor testing encountered the existing null queried-object warning in the deprecated listing-page content path, which is unchanged here.
• PHPUnit and the WordPress test library are unavailable locally. PHP 7.0.27 and the oldest supported WordPress versions were not runtime-tested.