feat(chat): send the client platform when starting a chat run

Web, iOS and Android now send X-Client-Platform (web, ios, android, the
same values their live stream already sends) on POST /chat, so the
backend can attribute AI chat reviews to the device. No other request
changes. The API gateway already allows the header, and a backend that
does not read it ignores it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: kirill-markin

apps/android/data/local/src/main/java/com/flashcardsopensourceapp/data/local/ai/remote/AiChatLiveRemoteService.kt

@@ -31,7 +31,7 @@ import java.util.concurrent.atomic.AtomicBoolean
 const val aiChatLiveStreamEndedBeforeTerminalCode: String = "ai_live_stream_ended_before_terminal"
 const val aiChatLiveStreamReadFailedCode: String = "ai_live_stream_read_failed"
 
-private const val aiChatLiveClientPlatform: String = "android"
+internal const val aiChatClientPlatform: String = "android"
 
 private const val aiChatLiveAttachThrottleStatusCode: Int = 429
 
@@ -227,7 +227,7 @@ class AiChatLiveRemoteService private constructor(
             .cacheControl(CacheControl.FORCE_NETWORK)
             .header("Accept", "text/event-stream")
             .header("Authorization", authorization)
-            .header("X-Client-Platform", aiChatLiveClientPlatform)
+            .header("X-Client-Platform", aiChatClientPlatform)
             .header("X-Chat-Live-Client-Id", aiChatLiveClientId)
         val clientVersion = observationVersions.clientVersion
         if (clientVersion != null) {

apps/android/data/local/src/main/java/com/flashcardsopensourceapp/data/local/ai/remote/AiChatRemoteService.kt

@@ -388,7 +388,7 @@ class AiChatRemoteService private constructor(
                 method = "POST",
                 authorizationHeader = authorizationHeader,
                 requestBody = requestJson.toRequestBody(aiJsonMediaType),
-                extraHeaders = emptyMap()
+                extraHeaders = mapOf("X-Client-Platform" to aiChatClientPlatform)
             )
         )
         return@withContext decodeAiChatStartRunResponse(responseBody)

apps/ios/Flashcards/Flashcards/AI/Runtime/AIChatService.swift

@@ -370,7 +370,8 @@ final class AIChatService: AIChatSessionServicing, @unchecked Sendable {
             path: "/chat",
             method: "POST",
             bodyData: encodedBody,
-            clientRequestId: clientRequestId
+            clientRequestId: clientRequestId,
+            additionalHeaders: ["X-Client-Platform": aiChatClientPlatform]
         )
         let data = try await self.execute(
             session: session,
@@ -606,14 +607,15 @@ final class AIChatService: AIChatSessionServicing, @unchecked Sendable {
         path: String,
         method: String,
         bodyData: Data,
-        clientRequestId: String
+        clientRequestId: String,
+        additionalHeaders: [String: String]
     ) throws -> URLRequest {
         var request = try self.makeRequest(
             session: session,
             path: path,
             method: method,
             clientRequestId: clientRequestId,
-            additionalHeaders: [:]
+            additionalHeaders: additionalHeaders
         )
         request.setValue("application/json", forHTTPHeaderField: "Content-Type")
         request.httpBody = bodyData

apps/web/src/api/endpoints/chat.ts

@@ -68,6 +68,9 @@ export async function getChatSnapshotWithResumeDiagnostics(
 export async function startChatRun(body: StartChatRunRequestBody): Promise<StartChatRunResponse> {
   return parseContractResponse(await requestJson("/chat", {
     method: "POST",
+    headers: {
+      "X-Client-Platform": "web",
+    },
     body: JSON.stringify(body),
   }, allowAuthRecoveryWithTransientNetworkRetry), "POST /chat", parseStartChatRunResponse);
 }