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.

tests/Unit/RegistryDriverTest.php

@@ -0,0 +1,68 @@
+<?php
+
+use InvoiceShelf\Modules\Registry;
+
+afterEach(function () {
+    Registry::flushDrivers();
+});
+
+test('registerDriver stores driver metadata under its type', function () {
+    Registry::flushDrivers();
+
+    Registry::registerDriver('exchange_rate', 'fake_provider', [
+        'class' => 'FakeDriver',
+        'label' => 'fake.label',
+    ]);
+
+    expect(Registry::driverMeta('exchange_rate', 'fake_provider'))
+        ->toEqual([
+            'class' => 'FakeDriver',
+            'label' => 'fake.label',
+        ]);
+});
+
+test('registerExchangeRateDriver delegates to registerDriver with the exchange_rate type', function () {
+    Registry::flushDrivers();
+
+    Registry::registerExchangeRateDriver('fake_provider', [
+        'class' => 'FakeDriver',
+        'label' => 'fake.label',
+    ]);
+
+    expect(Registry::driverMeta('exchange_rate', 'fake_provider'))
+        ->not->toBeNull()
+        ->and(Registry::allDrivers('exchange_rate'))
+        ->toHaveKey('fake_provider');
+});
+
+test('allDrivers returns an empty array for unknown types', function () {
+    Registry::flushDrivers();
+
+    expect(Registry::allDrivers('nonexistent'))->toBe([]);
+});
+
+test('driverMeta returns null for unknown drivers', function () {
+    Registry::flushDrivers();
+
+    expect(Registry::driverMeta('exchange_rate', 'nonexistent'))->toBeNull();
+});
+
+test('flushDrivers clears all driver registrations', function () {
+    Registry::registerExchangeRateDriver('a', ['class' => 'A', 'label' => 'a']);
+    Registry::registerExchangeRateDriver('b', ['class' => 'B', 'label' => 'b']);
+
+    Registry::flushDrivers();
+
+    expect(Registry::allDrivers('exchange_rate'))->toBe([]);
+});
+
+test('flush does not clear driver registrations', function () {
+    Registry::registerExchangeRateDriver('persists', [
+        'class' => 'PersistDriver',
+        'label' => 'persist.label',
+    ]);
+
+    Registry::flush();
+
+    expect(Registry::driverMeta('exchange_rate', 'persists'))->not->toBeNull();
+});