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

crates/skald-core/src/chatbot/mod.rs

@@ -2,6 +2,6 @@ pub mod logging;
 
 // Re-export from the independent llm-client crate.
 pub use llm_client::{
-    ChatOptions, ChatResponse, ChatbotClient, LlmError, LlmRawMeta, LlmTurn, Message, ToolCall,
-    anthropic, http_status, lm_studio, ollama, openai,
+    ChatOptions, ChatResponse, ChatbotClient, LlmError, LlmRawMeta, LlmTurn, Message, StreamDelta,
+    ToolCall, anthropic, http_status, lm_studio, ollama, openai,
 };