perf: bump default chunk_size 1000 -> 6000 + log chunk stats

Root cause of 24-min translation: frontend default chunk_size was 1000
(upstream xunbu default), producing 78 requests for the client's WI
document (verified by running segments2json_chunks on the real file:
1000 -> 78 chunks, matches server log's '75 requests').

- index.html: default chunk_size 1000 -> 6000 (78 -> 13 requests, 6x
  fewer). 6000 is safe vs qwen3.6-plus 8-16K output limit, and gives
  the LLM MORE context (better terminology consistency). Slider range
  already allows up to 12000.
- segments_agent.py: log segment count / total chars / chunk_size /
  expected request count after chunking, so 'slow translation' reports
  can be diagnosed instantly (doc structure vs chunk size vs API latency).

docutranslate/agents/segments_agent.py

@@ -323,6 +323,10 @@ class SegmentsTranslateAgent(Agent):
 
         # Non-MT mode: JSON batch translation
         indexed_originals, chunks, merged_indices_list = segments2json_chunks(segments, chunk_size)
+        self.logger.info(
+            f"分块统计: 段数={len(segments)}, 总字符={sum(len(s) for s in segments)}, "
+            f"chunk_size={chunk_size}, 预计请求={len(chunks)}"
+        )
         prompts = [generate_prompt(json.dumps(chunk, ensure_ascii=False, indent=0), self.to_lang) for chunk in chunks]
         translated_chunks = super().send_prompts(prompts=prompts, json_format=self.force_json,
                                                  pre_send_handler=self._pre_send_handler,
@@ -414,6 +418,10 @@ class SegmentsTranslateAgent(Agent):
         # Non-MT mode: JSON batch translation
         indexed_originals, chunks, merged_indices_list = await asyncio.to_thread(segments2json_chunks, segments,
                                                                                  chunk_size)
+        self.logger.info(
+            f"分块统计: 段数={len(segments)}, 总字符={sum(len(s) for s in segments)}, "
+            f"chunk_size={chunk_size}, 预计请求={len(chunks)}"
+        )
         prompts = [generate_prompt(json.dumps(chunk, ensure_ascii=False, indent=0), self.to_lang) for chunk in chunks]
         translated_chunks = await super().send_prompts_async(prompts=prompts, force_json=self.force_json,
                                                              pre_send_handler=self._pre_send_handler,

docutranslate/static/index.html

@@ -1120,7 +1120,7 @@
                 custom_to_lang: '',
                 thinking: 'disable',
                 custom_prompt: '',
-                chunk_size: 1000,
+                chunk_size: 6000,
                 concurrent: 5,
                 temperature: 0.1,
                 retry: 3,
@@ -1187,7 +1187,7 @@
                 form.custom_to_lang = get('translator_custom_to_lang', '');
                 form.thinking = get('translator_thinking_mode', 'default');
                 form.custom_prompt = get('custom_prompt', '');
-                form.chunk_size = getNum('chunk_size', 1000);
+                form.chunk_size = getNum('chunk_size', 6000);
                 form.concurrent = getNum('concurrent', 5);
                 form.temperature = getNum('temperature', 0.1);
                 form.retry = getNum('retry', 3);