feat(exchange-rate): make providers extendible via module Registry

Exchange rate providers are now pluggable via the module Registry. The four built-in drivers (currency_converter, currency_freak, currency_layer, open_exchange_rate) move from a static config array into App\\Providers\\DriverRegistryProvider, which calls Registry::registerExchangeRateDriver() for each during app boot with metadata the frontend needs: label (i18n key), website (help-text URL), and config_fields (schema for driver-specific driver_config JSON).

The Currency Converter's server-type selector and dedicated URL field — previously hardcoded in ExchangeRateProviderModal.vue — are now just another config_fields entry with a visible_when rule that shows the URL input only when type=DEDICATED. Any module that wants to ship a custom driver gets the same treatment for free: declare config_fields in the registration, and the host app's modal renders them automatically.

ExchangeRateDriverFactory::make() falls back to Registry::driverMeta() when a name isn't in the local built-in map, and availableDrivers() merges both sources. ConfigController handles the exchange_rate_drivers key specially by mapping Registry::allDrivers('exchange_rate') to enriched option objects, so the config-file route still works for every other key. The static exchange_rate_drivers + currency_converter_servers arrays in config/invoiceshelf.php are deleted.

Unit tests cover the new Registry::register/flushDrivers, the factory merging built-ins with Registry-contributed drivers, and the factory rejecting unknown names. A feature test exercises the end-to-end /api/v1/config?key=exchange_rate_drivers response shape.

NOTE: this commit depends on invoiceshelf/modules package commit e44d951 which adds the Registry driver API. The package needs to be released and pinned in composer.json before a fresh composer install on this commit will work.

app/Http/Controllers/Company/General/ConfigController.php

@@ -3,20 +3,47 @@
 namespace App\Http\Controllers\Company\General;
 
 use App\Http\Controllers\Controller;
+use Illuminate\Http\JsonResponse;
 use Illuminate\Http\Request;
-use Illuminate\Http\Response;
+use InvoiceShelf\Modules\Registry;
 
 class ConfigController extends Controller
 {
     /**
      * Handle the incoming request.
-     *
-     * @return Response
      */
-    public function __invoke(Request $request)
+    public function __invoke(Request $request): JsonResponse
     {
+        if ($request->key === 'exchange_rate_drivers') {
+            return response()->json([
+                'exchange_rate_drivers' => $this->exchangeRateDrivers(),
+            ]);
+        }
+
         return response()->json([
             $request->key => config('invoiceshelf.'.$request->key),
         ]);
     }
+
+    /**
+     * Build the exchange rate driver list from the module Registry.
+     *
+     * Returns enriched objects (with label, website, and config_fields) so the
+     * frontend can render driver-specific configuration forms without hardcoding
+     * any per-driver UI.
+     *
+     * @return array<int, array<string, mixed>>
+     */
+    protected function exchangeRateDrivers(): array
+    {
+        return collect(Registry::allDrivers('exchange_rate'))
+            ->map(fn (array $meta, string $name) => [
+                'value' => $name,
+                'label' => $meta['label'] ?? $name,
+                'website' => $meta['website'] ?? '',
+                'config_fields' => $meta['config_fields'] ?? [],
+            ])
+            ->values()
+            ->all();
+    }
 }