refactor(web): load channel configs without exposing secret values (#2277)

* refactor(web): load channel configs without exposing secret values

- add a dedicated channel config API that returns sanitized config plus
  configured secret metadata
- update channel config pages and forms to use secret presence for
  placeholders, validation, reset, and save behavior
- refresh the channel settings layout and clean up related i18n copy
- add backend tests for the new channel config endpoint

* fix(config): restore missing strings import

web/frontend/src/api/channels.ts

@@ -1,5 +1,3 @@
-// API client for channels navigation and channel-specific config flows.
-
 import { launcherFetch } from "@/api/http"
 
 export type ChannelConfig = Record<string, unknown>
@@ -12,6 +10,13 @@ export interface SupportedChannel {
   variant?: string
 }
 
+export interface ChannelConfigResponse {
+  config: ChannelConfig
+  configured_secrets: string[]
+  config_key: string
+  variant?: string
+}
+
 interface ChannelsCatalogResponse {
   channels: SupportedChannel[]
 }
@@ -54,6 +59,14 @@ export async function getAppConfig(): Promise<AppConfig> {
   return request<AppConfig>("/api/config")
 }
 
+export async function getChannelConfig(
+  channelName: string,
+): Promise<ChannelConfigResponse> {
+  return request<ChannelConfigResponse>(
+    `/api/channels/${encodeURIComponent(channelName)}/config`,
+  )
+}
+
 export async function patchAppConfig(
   patch: Record<string, unknown>,
 ): Promise<ConfigActionResponse> {