Fix template rendering issue and improve export functionality

- Fix template rendering by storing rendered HTML in variable before writing
  This resolves subdirectories not appearing in navigation despite being
  correctly detected and included in template variables
- Add missing template variables (paths.work_dir) to gallery config
- Add CGI refresh handler for web-based gallery regeneration
- Improve PDF export functionality with better error handling
- Add ESC key shortcut documentation for exiting selection mode
- Enhanced export manager with proper dependency checking

Fixes issue where new subdirectories were detected but not displayed
in browser navigation due to incomplete template rendering.

templates/gallery.html

@@ -108,6 +108,10 @@
       <span>Export plots</span>
       <span class="shortcut-key">Ctrl+E</span>
     </div>
+    <div class="shortcut-item">
+      <span>Exit selection mode</span>
+      <span class="shortcut-key">Esc</span>
+    </div>
     <div class="shortcut-item">
       <span>Refresh</span>
       <span class="shortcut-key">F5</span>
@@ -186,6 +190,7 @@
     window.galleryConfig = {
       searchDebounceMs: {{ ui.search_debounce_ms|default(300) }},
       maxRecentPlots: {{ ui.max_recent_plots|default(20) }},
+      workDir: "{{ paths.work_dir }}",
       stats: {% if stats %}{{ stats|tojson }}{% else %}null{% endif %}
     };
   </script>