feat: Add multi-language support (i18n)

Languages supported (8):
- English (en)
- 日本語 (ja)
- Deutsch (de)
- Français (fr)
- Español (es)
- 简体中文 (zh-CN)
- 繁體中文 (zh-TW)
- 한국어 (ko)

Changes:
- Add locale column to users table
- Add SetLocale middleware for automatic locale detection
- Add LocaleController for language switching
- Create language files with translations for all UI elements
- Add language selector to user profile page
- Update all Blade views to use translation strings

src/resources/views/profile/partials/update-locale-form.blade.php

@@ -0,0 +1,46 @@
+<section>
+    <header>
+        <h2 class="text-lg font-medium text-gray-900">
+            {{ __('messages.settings.language') }}
+        </h2>
+
+        <p class="mt-1 text-sm text-gray-600">
+            {{ __('messages.settings.select_language') }}
+        </p>
+    </header>
+
+    <form method="post" action="{{ route('locale.update') }}" class="mt-6 space-y-6">
+        @csrf
+
+        <div>
+            <x-input-label for="locale" :value="__('messages.settings.language')" />
+            <select
+                id="locale"
+                name="locale"
+                class="mt-1 block w-full border-gray-300 focus:border-indigo-500 focus:ring-indigo-500 rounded-md shadow-sm"
+            >
+                @foreach(\App\Http\Middleware\SetLocale::SUPPORTED_LOCALES as $code => $name)
+                    <option value="{{ $code }}" {{ (auth()->user()->locale ?? 'en') === $code ? 'selected' : '' }}>
+                        {{ $name }}
+                    </option>
+                @endforeach
+            </select>
+            <x-input-error class="mt-2" :messages="$errors->get('locale')" />
+        </div>
+
+        <div class="flex items-center gap-4">
+            <x-primary-button>{{ __('messages.common.save') }}</x-primary-button>
+
+            @if (session('success'))
+                <p
+                    x-data="{ show: true }"
+                    x-show="show"
+                    x-transition
+                    x-init="setTimeout(() => show = false, 2000)"
+                    class="text-sm text-gray-600"
+                >{{ session('success') }}</p>
+            @endif
+        </div>
+    </form>
+</section>
+