fix(ai-chat): surface dropped-stream errors clearly + log client disconnects

A mid-stream connection drop showed a generic "Something went wrong / Load
failed" banner and left no server-side trace.

- error-message: classify the browsers' own fetch-failure strings ("Load
  failed" on WebKit, "Failed to fetch" on Chrome, "NetworkError" on Firefox)
  as a lost connection, so the banner names the cause instead of the generic
  heading.
- ai-chat.controller: log a warning in the request close handler when the
  client disconnects before completion, so a drop that reaches the app (e.g. a
  reverse proxy cutting the SSE) is visible in the server logs before the abort.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/ai-chat/utils/error-message.ts

@@ -85,8 +85,12 @@ function classifyProviderError(msg: string): ErrorCategory | null {
   // Connection dropped / provider unreachable. ECONNRESET is the production case:
   // the LLM socket was reset mid-stream. "terminated" is scoped to a connection/
   // stream context so it does not match benign "... was terminated" messages.
+  // The browser's own fetch-failure messages also land here because they mean the
+  // SSE stream to /api/ai-chat/stream dropped mid-answer (e.g. a reverse proxy cut
+  // it): WebKit/Safari says "Load failed", Chrome "Failed to fetch", Firefox
+  // "NetworkError when attempting to fetch resource".
   if (
-    /ECONNRESET|ECONNREFUSED|ENOTFOUND|EAI_AGAIN|EPIPE|socket hang up|cannot connect|fetch failed|network error|connection (?:error|closed|reset|terminated)|stream terminated/i.test(
+    /ECONNRESET|ECONNREFUSED|ENOTFOUND|EAI_AGAIN|EPIPE|socket hang up|cannot connect|fetch failed|failed to fetch|load failed|networkerror|network error|connection (?:error|closed|reset|terminated)|stream terminated/i.test(
       head,
     )
   ) {

apps/server/src/core/ai-chat/ai-chat.controller.ts

@@ -161,7 +161,16 @@ export class AiChatController {
     // cannot simply remove it once `stream()` returns).
     const controller = new AbortController();
     const onClose = (): void => {
-      if (!res.raw.writableEnded) controller.abort();
+      // A genuine disconnect leaves the response unfinished (unlike a normal
+      // completion, which also fires `close`). Such a drop — e.g. a reverse
+      // proxy cutting the SSE mid-answer — is otherwise invisible server-side,
+      // so log it here before aborting the agent loop.
+      if (!res.raw.writableEnded) {
+        this.logger.warn(
+          'AI chat stream: client disconnected before completion; aborting turn',
+        );
+        controller.abort();
+      }
     };
     req.raw.once('close', onClose);
     res.raw.once('finish', () => req.raw.off('close', onClose));