feat(image-popup): add auto-popup for detected screenshots and images

Implements image file watching in session working directories with
automatic popup display in the web UI:

- Add ImageWatcher class using chokidar for cross-platform file watching
- Add ImageDetectedEvent type for SSE broadcasting
- Integrate with session lifecycle (watch on create, unwatch on delete)
- Add image:detected SSE event handler in frontend
- Create draggable image popup window with open-in-new-tab support
- Add CSS styling with compact wizard layout

Supports: png, jpg, jpeg, gif, webp, bmp, svg

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

src/web/public/index.html

@@ -942,7 +942,7 @@
 
     <!-- Ralph Loop Wizard Modal -->
     <div class="modal" id="ralphWizardModal">
-      <div class="modal-backdrop" onclick="app.closeRalphWizard()"></div>
+      <div class="modal-backdrop"></div>
       <div class="modal-content modal-wizard">
         <div class="modal-header">
           <h3>Start Ralph Loop</h3>
@@ -976,7 +976,7 @@
               <div class="textarea-with-assist">
                 <textarea
                   id="ralphTaskDescription"
-                  rows="4"
+                  rows="3"
                   placeholder="Build a REST API for user authentication with JWT tokens, login/logout endpoints, and password reset flow."></textarea>
               </div>
             </div>