feat(ui): blackflame theming pass — edit toggle, md tables, back button, Little Blue action cards

- markdown_editor Edit toggle uses themed ghost button
- .md-preview gets full blackflame styling incl. tables (migrated BookStack tables now render as tables)
- reusable back button on page/reference/project/resource reading views
- Little Blue actions regrouped into themed cards, pairing Start/Stop per guest

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/views/reference.js

@@ -1,6 +1,7 @@
 // Reference detail: media block + summary + metadata + tag attach/detach + linked-from.
 import { api } from '../api.js';
 import { el, mount, clear, safeHref } from '../dom.js';
+import { backButton } from '../components/backbtn.js';
 
 function mediaBlock(ref) {
   if (ref.kind === 'image' && (ref.source_url || ref.blob_path)) {
@@ -108,6 +109,7 @@ export async function render(main, ctx) {
   );
 
   mount(main,
+    backButton(),
     el('h1', { class: 'view-h1' }, ref.title || '(untitled reference)'),
     el('p', { class: 'view-sub' },
       el('span', { class: 'status idle' }, ref.kind),