server: capture failed (non-200) LLM requests (#862)

Store a request/response capture for non-200 responses so failed
requests can be inspected in the activity log's Capture dialog, matching
the existing behavior for successful requests.

- extract storeCapture/decodeResponseBody helpers to share capture logic
between the success and non-200 paths
- record non-200 bodies (decompressed) so error details are viewable
- the activity UI already gates the View button on has_capture, so it
now appears for failed requests with no UI changes
- add tests for capturing failed requests and the disabled-captures case

closes #766

ui-svelte/src/lib/types.ts

@@ -43,6 +43,7 @@ export interface ActivityLogEntry {
   tokens: TokenMetrics;
   duration_ms: number;
   has_capture: boolean;
+  error_msg?: string;
   metadata?: Record<string, string>;
 }