token streaming & reasoning display: live SSE tokens frontend to back

- Add StreamDelta(SseDecoder) framing shared by OpenAI/Anthropic
- OpenAiClient: stream=true + reasoning_content deltas, index-based
  tool_calls accumulation, usage from final chunk
- AnthropicClient: message_start/content_block_*/message_delta events,
  thinking_delta->reasoning, input_json_delta->tool input
- TokenDelta ServerEvent variant wired through ChatHub + WS broadcast
- Frontend throttled flush (~15 Hz), pending bubble mutate-in-place,
  reasoning as collapsed-by-default <details>
- Drop streaming bubble on error/llm_failed/model_fallback
- i18n: chat.reasoning key added to en/fr/it

src/frontend/api/sessions.rs

@@ -633,6 +633,7 @@ fn build_items<'a>(
                             "failed":        failed,
                             "input_tokens":  msg.input_tokens,
                             "output_tokens": msg.output_tokens,
+                            "reasoning":     msg.reasoning_content,
                         }));
                     } else {
                         if !msg.content.trim().is_empty() {
@@ -643,6 +644,7 @@ fn build_items<'a>(
                                 "failed":        failed,
                                 "input_tokens":  msg.input_tokens,
                                 "output_tokens": msg.output_tokens,
+                                "reasoning":     msg.reasoning_content,
                             }));
                         }
                         for tc in &tool_calls {