Refactor UI, settings, and code quality across all categories

- Fix Replace Cover: routes now read preview_path from form POST instead of session (session writes from background threads were lost)
- Fix batch generation: submit all jobs immediately, poll all in parallel via Promise.all
- Fix label NameError in character generate route
- Fix style detail missing characters context
- Selected Preview pane: click any image to select it; data-preview-path on all images across all 8 detail templates
- Gallery → Library rename across all index page headings and navbar
- Settings: add configurable LoRA/checkpoint directories; default checkpoint selector moved from navbar to settings page
- Consolidate 6 get_available_*_loras() into single get_available_loras(category) reading from Settings
- ComfyUI tooltip shows currently loaded checkpoint name
- Remove navbar checkpoint bar
- Phase 4 cleanup: remove dead _queue_generation(), add session.modified, standardize log prefixes, rename action_type → action

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/gallery.html

@@ -2,7 +2,7 @@
 
 {% block content %}
 <div class="d-flex align-items-center justify-content-between mb-3">
-  <h4 class="mb-0">Gallery
+  <h4 class="mb-0">Image Gallery
     <span class="text-muted fs-6 fw-normal ms-2">{{ total }} image{{ 's' if total != 1 else '' }}</span>
   </h4>
 </div>