fix: auto-switch to new session now works correctly

Fixed bug where firstSessionId was set when i===1 instead of i===0,
causing it to never be set when creating a single session.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/web/public/app.js

@@ -756,7 +756,7 @@ class ClaudemanApp {
         });
 
         // Track first session
-        if (i === 1) {
+        if (i === 0) {
           firstSessionId = createData.session.id;
         }