fix(studio): improve chat context, thinking tags, streaming, and tool results

- Fix cleanThinkingTags to use proper regex instead of naive ReplaceAll
- Send conversation history (last 20 messages + summary) to AI instead of single message
- Store tool results alongside tool calls so history shows complete execution info
- Stream words instead of characters for smoother SSE rendering
- Add stop button to cancel in-progress AI requests (AbortController)
- Fix markdown rendering: add h2 support, use div for bullets
- Add i18n keys for cancel/stop (EN + FR)

💘 Generated with Crush

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

web/src/styles/global.css

@@ -752,6 +752,12 @@ input::placeholder { color: var(--text-disabled); }
 }
 .studio-send-btn:hover:not(:disabled) { background: var(--accent-bright); border-color: var(--accent-bright); }
 .studio-send-btn:disabled { opacity: 0.3; cursor: not-allowed; }
+.studio-stop-btn {
+  width: 42px; height: 42px; padding: 0; display: flex; align-items: center; justify-content: center;
+  border-radius: var(--radius); background: var(--error); color: #fff; border: 1px solid var(--error);
+  cursor: pointer; transition: all 0.15s; flex-shrink: 0;
+}
+.studio-stop-btn:hover { opacity: 0.8; }
 .studio-input-hint { font-size: 11px; color: var(--text-disabled); text-align: center; margin-top: 6px; }
 
 /* ── Studio Tool Blocks ── */