feat: production routing support for subdomain and custom domain modes

Double-mount store routes at /store/* and /store/{store_code}/* so the
same handlers work in dev path-based, prod path-based, prod subdomain,
and prod custom-domain modes.  Wire StorePlatform.custom_subdomain into
StoreContextMiddleware for per-platform subdomain overrides.  Add admin
custom-domain management UI, fix stale /shop/ reset link, add
/merchants/ to reserved paths, and server-render window.STORE_CODE for
JS that previously parsed the URL.

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

app/modules/messaging/routes/pages/store.py

@@ -12,7 +12,11 @@ from fastapi import APIRouter, Depends, Path, Request
 from fastapi.responses import HTMLResponse
 from sqlalchemy.orm import Session
 
-from app.api.deps import get_current_store_from_cookie_or_header, get_db
+from app.api.deps import (
+    get_current_store_from_cookie_or_header,
+    get_db,
+    get_resolved_store_code,
+)
 from app.modules.core.utils.page_context import get_store_context
 from app.modules.tenancy.models import User
 from app.templates_config import templates
@@ -26,11 +30,11 @@ router = APIRouter()
 
 
 @router.get(
-    "/{store_code}/messages", response_class=HTMLResponse, include_in_schema=False
+    "/messages", response_class=HTMLResponse, include_in_schema=False
 )
 async def store_messages_page(
     request: Request,
-    store_code: str = Path(..., description="Store code"),
+    store_code: str = Depends(get_resolved_store_code),
     current_user: User = Depends(get_current_store_from_cookie_or_header),
     db: Session = Depends(get_db),
 ):
@@ -45,13 +49,13 @@ async def store_messages_page(
 
 
 @router.get(
-    "/{store_code}/messages/{conversation_id}",
+    "/messages/{conversation_id}",
     response_class=HTMLResponse,
     include_in_schema=False,
 )
 async def store_message_detail_page(
     request: Request,
-    store_code: str = Path(..., description="Store code"),
+    store_code: str = Depends(get_resolved_store_code),
     conversation_id: int = Path(..., description="Conversation ID"),
     current_user: User = Depends(get_current_store_from_cookie_or_header),
     db: Session = Depends(get_db),
@@ -74,13 +78,13 @@ async def store_message_detail_page(
 
 
 @router.get(
-    "/{store_code}/email-templates",
+    "/email-templates",
     response_class=HTMLResponse,
     include_in_schema=False,
 )
 async def store_email_templates_page(
     request: Request,
-    store_code: str = Path(..., description="Store code"),
+    store_code: str = Depends(get_resolved_store_code),
     current_user: User = Depends(get_current_store_from_cookie_or_header),
     db: Session = Depends(get_db),
 ):