fix(assets): only enqueue context-specific assets on LifterLMS pages

[faisalahammad]

Description

LifterLMS currently enqueues a set of frontend scripts and styles on every page where the plugin is active, even plain posts and pages that contain no LifterLMS content. This change conditionally enqueues the context-specific assets so they only load on LifterLMS-related pages (course, lesson, membership, account, checkout).

Affected assets (now gated):

• jquery-ui-tooltip, jquery-ui-datepicker, jquery-ui-slider
• webui-popover (script and style)
• llms-ajax
• llms-form-checkout (narrowed to account and checkout only)

The lifterlms-styles stylesheet and the llms script remain enqueued by default for backward compatibility, but both can be suppressed through the new llms_load_frontend_assets filter.

Adds the llms_load_frontend_assets filter for sites that want to force assets on or off:

• add_filter( 'llms_load_frontend_assets', '__return_true' ); forces a load on pages the auto-detection misses (e.g. a custom template embedding LifterLMS shortcodes).
• add_filter( 'llms_load_frontend_assets', '__return_false' ); fully suppresses frontend assets for non-LifterLMS templates.

Shortcodes that render LifterLMS UI off the auto-detected page types ([lifterlms_courses], [lifterlms_memberships], [lifterlms_pricing_table], [lifterlms_checkout]) now self-enqueue lifterlms-styles so they remain styled when the filter is forced off. The pricing table shortcode also self-enqueues webui-popover so locked-plan popovers continue to work.

Fixes #146

How has this been tested?

Unit tests cover all gating paths in tests/phpunit/unit-tests/class-llms-test-frontend-assets.php:

• Context assets not enqueued on a plain (non-LifterLMS) page.
• Context assets enqueued on a single course page.
• llms-form-checkout enqueued on account and checkout pages.
• llms_load_frontend_assets filter forces load on a plain page.
• llms_load_frontend_assets filter suppresses gated assets on an LLMS page.
• Core assets (lifterlms-styles, llms) stay always-on by default.

Test run: composer tests -- --filter LLMS_Test_Frontend_Assets (19/19 pass).
Coding standards: composer check-cs (clean).

Manual testing covered:

• Plain page, course page, account page, checkout page source-viewed to confirm expected assets are present or absent.
• [lifterlms_pricing_table] on a plain page with a locked access plan: popover displays on click.
• [lifterlms_courses] on a plain page: course loop is styled.
• [lifterlms_checkout] shortcode on a page other than the configured checkout: form-checkout script loads.

Environment: WordPress 6.x, PHP 7.4/8.x.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality) - the llms_load_frontend_assets filter.

Checklist:

• This PR requires and contains at least one changelog file.
• My code has been tested.
• My code passes all existing automated tests.
• My code follows the LifterLMS Coding & Documentation Standards.