fix: Add Warning Messages About Invalid License Key and Usage Exceed

[Artem-Okladov]

Summary

• Added two branded warning messages on the WProofreader settings page: usage limit exceeded and invalid license key, shown when the WebSpellChecker service returns a 403 while fetching the live language list.
• Both messages render as Moodle warning notifications and are prefixed with a warning emoji.
• The live service check, and the warnings it can trigger, only runs on the WProofreader settings page itself, not on other Site administration pages.

Changes

• lang/en/local_wproofreader.php: added usage_limit_exceeded and service_id_invalid strings.
• classes/local/config_builder.php: pass usageLimitExceededMessage and serviceIdInvalidMessage through to the JS config.
• amd/src/settings_page.js: on a 403 from the service, show the usage-limit message when the error mentions quota, otherwise show the invalid-license message.
• classes/local/settings_page_heading.php (new): admin_setting_heading subclass whose output_html() queues the settings-page AMD module only when the WProofreader settings page is the one being rendered.
• settings.php: gate the setting definitions behind $ADMIN->fulltree (matches the pattern used by Moodle core plugins, e.g. blocks/rss_client) and use the new heading class to trigger the language-list check.
• amd/build/settings_page.min.js and .map: rebuilt via Moodle's grunt task.

Testing

Manual testing on Moodle 4.5 & 5.2 setups.