feat: add mixed-channel precheck API for account-group binding

Add a dedicated CheckMixedChannel endpoint that allows the frontend
to pre-validate mixed channel risk before submitting create/update
requests. This improves UX by showing warnings earlier in the flow
instead of only after form submission.

Backend changes:
- Add CheckMixedChannelRequest struct and CheckMixedChannel handler
- Register POST /check-mixed-channel route
- Expose CheckMixedChannelRisk as public method on AdminService
- Simplify Create/Update 409 responses (remove details/require_confirmation)
- Add comprehensive handler tests and stub methods

Frontend changes:
- Add checkMixedChannelRisk API function and TypeScript types
- Refactor CreateAccountModal to precheck before step transition and submission
- Refactor EditAccountModal to precheck before update submission
- Replace pendingPayload pattern with action-based dialog flow

backend/internal/server/routes/admin.go

@@ -219,6 +219,7 @@ func registerAccountRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		accounts.GET("", h.Admin.Account.List)
 		accounts.GET("/:id", h.Admin.Account.GetByID)
 		accounts.POST("", h.Admin.Account.Create)
+		accounts.POST("/check-mixed-channel", h.Admin.Account.CheckMixedChannel)
 		accounts.POST("/sync/crs", h.Admin.Account.SyncFromCRS)
 		accounts.POST("/sync/crs/preview", h.Admin.Account.PreviewFromCRS)
 		accounts.PUT("/:id", h.Admin.Account.Update)