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

frontend/src/types/index.ts

@@ -766,6 +766,26 @@ export interface UpdateAccountRequest {
   confirm_mixed_channel_risk?: boolean
 }
 
+export interface CheckMixedChannelRequest {
+  platform: AccountPlatform
+  group_ids: number[]
+  account_id?: number
+}
+
+export interface MixedChannelWarningDetails {
+  group_id: number
+  group_name: string
+  current_platform: string
+  other_platform: string
+}
+
+export interface CheckMixedChannelResponse {
+  has_risk: boolean
+  error?: string
+  message?: string
+  details?: MixedChannelWarningDetails
+}
+
 export interface CreateProxyRequest {
   name: string
   protocol: ProxyProtocol