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

web/components/copilot-render.js

@@ -507,6 +507,21 @@ export function renderAttachmentChips(host, attachments, { removable = false } =
     </div>`;
 }
 
+/**
+ * Collapsible chain-of-thought block: small, muted, collapsed by default so it
+ * never weighs on the UI. A native <details> — Lit keeps the element stable
+ * across re-renders, so a user-expanded block stays open while tokens stream
+ * into it (live) and in past history items alike.
+ */
+function renderReasoning(msg) {
+  if (!msg.reasoning) return nothing;
+  return html`
+    <details class="reasoning-block ${msg.streaming ? 'reasoning-block--live' : ''}">
+      <summary>${t('chat.reasoning')}</summary>
+      <div class="reasoning-content">${msg.reasoning}</div>
+    </details>`;
+}
+
 export function renderMsg(host, msg) {
   try {
     switch (msg.kind) {
@@ -516,6 +531,7 @@ export function renderMsg(host, msg) {
         return html`
           <div class="copilot-msg assistant copilot-markdown ${msg.failed ? 'copilot-msg--failed' : ''}">
             ${msg.failed ? failedBadge() : nothing}
+            ${renderReasoning(msg)}
             ${unsafeHTML(renderMarkdown(msg.content))}
             ${msg.input_tokens != null ? html`<div class="copilot-token-count">↑${msg.input_tokens.toLocaleString()} tok &nbsp;↓${msg.output_tokens?.toLocaleString()} tok</div>` : nothing}
           </div>`;
@@ -523,8 +539,10 @@ export function renderMsg(host, msg) {
         return html`
           <div class="copilot-msg assistant copilot-markdown ${msg.failed ? 'copilot-msg--failed' : ''}">
             ${msg.failed ? failedBadge() : nothing}
+            ${renderReasoning(msg)}
             ${unsafeHTML(renderMarkdown(msg.content))}
-            ${msg.input_tokens != null ? html`<div class="copilot-token-count">↑${msg.input_tokens.toLocaleString()} tok &nbsp;↓${msg.output_tokens?.toLocaleString()} tok</div>` : nothing}
+            ${msg.streaming ? html`<span class="stream-caret"></span>` : nothing}
+            ${msg.input_tokens != null && !msg.streaming ? html`<div class="copilot-token-count">↑${msg.input_tokens.toLocaleString()} tok &nbsp;↓${msg.output_tokens?.toLocaleString()} tok</div>` : nothing}
           </div>`;
       case 'error':
         return html`