feat: add merchant user edit page with editable profile fields

- Add /admin/merchant-users/{id}/edit page route and template
- Replace toggle-status button with edit button on merchant-users list
- Editable fields: username, email, first name, last name
- Quick actions: toggle status, delete (with double confirm)
- Move RBAC two-phase plan to docs/proposals/

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

app/modules/tenancy/routes/pages/admin.py

@@ -288,6 +288,29 @@ async def admin_merchant_user_detail_page(
     )
 
 
+@router.get(
+    "/merchant-users/{user_id}/edit",
+    response_class=HTMLResponse,
+    include_in_schema=False,
+)
+async def admin_merchant_user_edit_page(
+    request: Request,
+    user_id: int = Path(..., description="User ID"),
+    current_user: User = Depends(
+        require_menu_access("merchant-users", FrontendType.ADMIN)
+    ),
+    db: Session = Depends(get_db),
+):
+    """
+    Render merchant user edit form.
+    Allows editing merchant owner or store team member details.
+    """
+    return templates.TemplateResponse(
+        "tenancy/admin/merchant-user-edit.html",
+        get_admin_context(request, db, current_user, user_id=user_id),
+    )
+
+
 # ============================================================================
 # ADMIN USER MANAGEMENT ROUTES (Super Admin Only)
 # ============================================================================