feat: add setting to show subagent windows for active tab only

Adds "Subagents for Active Tab Only" toggle in App Settings → Display:
- OFF (default): windows visible regardless of active tab (original behavior)
- ON: windows only visible when their parent tab is selected

Also improves parent session detection by checking active session first,
which fixes connection lines going to wrong tabs when multiple sessions
share the same working directory.

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

src/web/public/index.html

@@ -630,6 +630,14 @@
               </label>
               <span class="form-hint">Show the Subagents panel (independent from Monitor)</span>
             </div>
+            <div class="form-row form-row-switch">
+              <label>Subagents for Active Tab Only</label>
+              <label class="switch">
+                <input type="checkbox" id="appSettingsSubagentActiveTabOnly">
+                <span class="slider"></span>
+              </label>
+              <span class="form-hint">Only show subagent windows when their parent tab is selected</span>
+            </div>
             <div class="form-row form-row-switch">
               <label>Show Project Insights Panel</label>
               <label class="switch">