feat: multi-module improvements across merchant, store, i18n, and customer systems

- Fix platform-grouped merchant sidebar menu with core items at root level
- Add merchant store management (detail page, create store, team page)
- Fix store settings 500 error by removing dead stripe/API tab
- Move onboarding translations to module-owned locale files
- Fix onboarding banner i18n with server-side rendering + context inheritance
- Refactor login language selectors to use languageSelector() function (LANG-002)
- Move HTTPException handling to global exception handler in merchant routes (API-003)
- Add language selector to all login pages and portal headers
- Fix customer module: drop order stats from customer model, add to orders module
- Fix admin menu config visibility for super admin platform context
- Fix storefront auth and layout issues
- Add missing i18n translations for onboarding steps (en/fr/de/lb)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/customers/migrations/versions/customers_002_drop_order_stats.py

@@ -0,0 +1,35 @@
+"""customers 002 - drop order stats columns (moved to orders module)
+
+Revision ID: customers_002
+Revises: customers_001
+Create Date: 2026-03-07
+"""
+import sqlalchemy as sa
+
+from alembic import op
+
+revision = "customers_002"
+down_revision = "customers_001"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.drop_column("customers", "total_orders")
+    op.drop_column("customers", "total_spent")
+    op.drop_column("customers", "last_order_date")
+
+
+def downgrade() -> None:
+    op.add_column(
+        "customers",
+        sa.Column("last_order_date", sa.DateTime(), nullable=True),
+    )
+    op.add_column(
+        "customers",
+        sa.Column("total_spent", sa.Numeric(10, 2), nullable=True, server_default="0"),
+    )
+    op.add_column(
+        "customers",
+        sa.Column("total_orders", sa.Integer(), nullable=True, server_default="0"),
+    )