Consolidate Admin Settings controllers: merge Get/Update pairs

Merge GetCompanySettingsController + UpdateCompanySettingsController into
CompanySettingsController with show() and update() methods.

Merge GetUserSettingsController + UpdateUserSettingsController into
UserSettingsController with show() and update() methods.

Absorb GetCompanyMailConfigurationController into
CompanyMailConfigurationController as getDefaultConfig() method.

Removes 5 single-action controllers, down to 4 Settings controllers.

app/Http/Controllers/V1/Admin/Settings/CompanySettingsController.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace App\Http\Controllers\V1\Admin\Settings;
+
+use App\Http\Controllers\Controller;
+use App\Http\Requests\GetSettingsRequest;
+use App\Http\Requests\UpdateSettingsRequest;
+use App\Models\Company;
+use App\Models\CompanySetting;
+use Illuminate\Http\JsonResponse;
+use Illuminate\Support\Arr;
+
+class CompanySettingsController extends Controller
+{
+    public function show(GetSettingsRequest $request): JsonResponse
+    {
+        $settings = CompanySetting::getSettings((array) $request->settings, $request->header('company'));
+
+        return response()->json($settings);
+    }
+
+    public function update(UpdateSettingsRequest $request): JsonResponse
+    {
+        $company = Company::find($request->header('company'));
+        $this->authorize('manage company', $company);
+
+        $data = $request->settings;
+
+        if (
+            Arr::exists($data, 'currency') &&
+            (CompanySetting::getSetting('currency', $company->id) !== $data['currency']) &&
+            $company->hasTransactions()
+        ) {
+            return response()->json([
+                'success' => false,
+                'message' => 'Cannot update company currency after transactions are created.',
+            ]);
+        }
+
+        CompanySetting::setSettings($data, $request->header('company'));
+
+        return response()->json([
+            'success' => true,
+        ]);
+    }
+}