feat(plugins): add sale document view hooks, webhook CSRF exemption, and WhatsApp plugin

Add three missing plugin hook points for sale documents so plugins can
inject buttons into invoice, quote, and work order views alongside the
existing receipt hook. All four pass ['saleId' => $sale_id_num] and
follow the same naming pattern (view:sales_{type}_buttons).

Exempt plugins/*/webhook from CSRF filtering to support server-to-server
provider callbacks. Also convert the CSRF except list from a string to an
array — the previous 'login|migrate' string produced a single unanchored
pattern, making login/anything CSRF-exempt. Separate array entries anchor
each one individually. Plugin webhook handlers are responsible for their
own authentication.

Add WhatsApp Business Cloud API plugin (app/Plugins/WhatsAppPlugin/):
- Free-form messaging page registered as the 'whatsapp' office module
  with its own permission, plus per-customer modal and thread view
- "Send via WhatsApp" button on all four sale document types via the new
  hooks; renders only when the customer has a phone number
- PDF delivery using the same sales/{type}_email view core uses for email
- Inbound webhook at plugins/whatsapp/webhook authenticated by
  X-Hub-Signature-256 HMAC; fails closed on missing/bad signature;
  always returns 200 to suppress Meta retries
- Out-of-order status callbacks cannot downgrade sent → delivered → read
- Conversation log table via plugin migration; dropped on uninstall with
  version reset so re-install recreates it cleanly
- Access token and app secret encrypted at rest in plugin_config; no
  writes to app_config or initial_schema.sql
- utf8mb4_unicode_520_ci collation throughout (MySQL and MariaDB compat)
- Language file stubs for all existing locales; English strings complete
- README covering credentials, install, webhook setup, and uninstall

app/Database/Migrations/sqlscripts/3.5.0_PluginConfigTableCreate.sql

@@ -9,7 +9,7 @@ CREATE TABLE IF NOT EXISTS `ospos_plugin_config` (
     PRIMARY KEY (`id`),
     UNIQUE KEY `uq_plugin_key` (`plugin_id`, `key`),
     KEY `idx_plugin_id` (`plugin_id`)
-) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_520_ci;
 
 INSERT IGNORE INTO `ospos_modules` (`name_lang_key`, `desc_lang_key`, `sort`, `module_id`) VALUES
     ('module_plugins', 'module_plugins_desc', 111, 'plugins');