Booster Plus for WooCommerce 7.2.4

* Fix - PRICES & CURRENCIES - Currency Exchange Rates - Crons - "Undefined index..." notices in log fixed.
* Dev - PRICES & CURRENCIES - Currency Exchange Rates - "Exchange Rates Offset (Percent)" options added for each currency pair.
* Dev - PRICES & CURRENCIES - Currency Exchange Rates - "Update all rates now" button added.
* Fix - PRICES & CURRENCIES - Multicurrency (Currency Switcher) - Session init moved to the `init` hook (on "WC Sessions").
* Fix - PRICES & CURRENCIES - Prices and Currencies by Country - Session init moved to the `init` hook (on "WC Sessions").
* Dev - PRODUCTS - Product Listings - Admin Settings - Descriptions updated.
* Dev - PRODUCTS - Product Tabs - "Content Processing" option added.
* Dev - EMAILS & MISC. - General - "Session Type in Booster" option re-enabled.
* Dev - Speed Optimization - "WC Sessions" is now the default option for `WCJ_SESSION_TYPE`. Affected modules: Checkout Files Upload, EU VAT Number, Multicurrency (Currency Switcher), Product Visibility by Country, Product Custom Visibility, Tax Display, Prices and Currencies by Country.
* Dev - Code refactoring - Replaced some code with `wcj_is_frontend()`.

* Fix - PDF INVOICING & PACKING SLIPS - `K_TCPDF_CALLS_IN_HTML` set to `true` (fixes the issue with using TCPDF methods in HTML, e.g. `[wcj_tcpdf_pagebreak]` shortcode).

* Dev - PDF INVOICING & PACKING SLIPS - Checking header image and background image to exist (before adding to PDF).
* Dev - PDF INVOICING & PACKING SLIPS - Header - Header Image - Admin description expanded.
* Dev - PDF INVOICING & PACKING SLIPS - Page Settings - Background Image - Admin description (with image path) added; admin description expanded.
* Dev - PDF INVOICING & PACKING SLIPS - Advanced - Default Images Directory - Default value set to `DOCUMENT_ROOT`.
* Dev - EMAILS & MISC. - Admin Tools - System Info - `$_SERVER` info added (works with `wcj_debug` param).