feat: add multi-tab feature to open 1-10 Claude sessions at once

- Add number input (1-10) next to "Run Claude" button in toolbar
- When tabCount > 1, create sessions named "1-projectname", "2-projectname", etc.
- All sessions share the same working directory (case folder)
- First session is auto-selected after creation
- Add CSS for small number input in toolbar

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

src/web/public/index.html

@@ -73,6 +73,7 @@
           <button class="btn-toolbar btn-primary" onclick="app.runClaude()" title="Run Claude (Ctrl+Enter)">
             Run Claude
           </button>
+          <input type="number" id="tabCount" class="toolbar-input-sm" value="1" min="1" max="10" title="Number of tabs to open (1-10)">
           <button class="btn-toolbar" onclick="app.runShell()" title="Run Shell">
             Shell
           </button>