feat: add multi-language (i18n) support for vendor dashboard and storefront

- Add database fields for language preferences:
  - Vendor: dashboard_language, storefront_language, storefront_languages
  - User: preferred_language
  - Customer: preferred_language

- Add language middleware for request-level language detection:
  - Cookie-based persistence
  - Browser Accept-Language fallback
  - Vendor storefront language constraints

- Add language API endpoints (/api/v1/language/*):
  - POST /set - Set language preference
  - GET /current - Get current language info
  - GET /list - List available languages
  - DELETE /clear - Clear preference

- Add i18n utilities (app/utils/i18n.py):
  - JSON-based translation loading
  - Jinja2 template integration
  - Language resolution helpers

- Add reusable language selector macros for templates
- Add languageSelector() Alpine.js component
- Add translation files (en, fr, de, lb) in static/locales/
- Add architecture rules documentation for language implementation
- Update marketplace-product-detail.js to use native language names

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

alembic/versions/fcfdc02d5138_add_language_settings_to_vendor_user_.py

@@ -0,0 +1,84 @@
+"""add_language_settings_to_vendor_user_customer
+
+Revision ID: fcfdc02d5138
+Revises: b412e0b49c2e
+Create Date: 2025-12-13 20:08:27.120863
+
+This migration adds language preference fields to support multi-language UI:
+- Vendor: default_language, dashboard_language, storefront_language
+- User: preferred_language
+- Customer: preferred_language
+
+Supported languages: en (English), fr (French), de (German), lb (Luxembourgish)
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'fcfdc02d5138'
+down_revision: Union[str, None] = 'b412e0b49c2e'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ========================================================================
+    # Vendor language settings
+    # ========================================================================
+    # default_language: Default language for vendor content (products, etc.)
+    op.add_column(
+        'vendors',
+        sa.Column('default_language', sa.String(5), nullable=False, server_default='fr')
+    )
+    # dashboard_language: Language for vendor team dashboard UI
+    op.add_column(
+        'vendors',
+        sa.Column('dashboard_language', sa.String(5), nullable=False, server_default='fr')
+    )
+    # storefront_language: Default language for customer-facing shop
+    op.add_column(
+        'vendors',
+        sa.Column('storefront_language', sa.String(5), nullable=False, server_default='fr')
+    )
+    # storefront_languages: JSON array of enabled languages for storefront
+    # Allows vendors to enable/disable specific languages
+    op.add_column(
+        'vendors',
+        sa.Column(
+            'storefront_languages',
+            sa.JSON,
+            nullable=False,
+            server_default='["fr", "de", "en"]'
+        )
+    )
+
+    # ========================================================================
+    # User language preference
+    # ========================================================================
+    # preferred_language: User's preferred UI language (NULL = use context default)
+    op.add_column(
+        'users',
+        sa.Column('preferred_language', sa.String(5), nullable=True)
+    )
+
+    # ========================================================================
+    # Customer language preference
+    # ========================================================================
+    # preferred_language: Customer's preferred language (NULL = use storefront default)
+    op.add_column(
+        'customers',
+        sa.Column('preferred_language', sa.String(5), nullable=True)
+    )
+
+
+def downgrade() -> None:
+    # Remove columns in reverse order
+    op.drop_column('customers', 'preferred_language')
+    op.drop_column('users', 'preferred_language')
+    op.drop_column('vendors', 'storefront_languages')
+    op.drop_column('vendors', 'storefront_language')
+    op.drop_column('vendors', 'dashboard_language')
+    op.drop_column('vendors', 'default_language')