fix: use Svelte dispatch for verstak-link events in MarkdownPreview

Problem: MarkdownPreview dispatched DOM CustomEvent via link.dispatchEvent()
which doesn't propagate through Svelte's event system. The on:verstak-link
handler on <MarkdownPreview> in NoteEditorPanel only catches Svelte dispatch()
events, not DOM CustomEvents from {@html} content.

Fix: Replaced DOM CustomEvent dispatch with Svelte createEventDispatcher.
Now handleClick() in MarkdownPreview calls dispatch('verstak-link', {...})
which properly propagates through NoteEditorPanel → App.svelte chain.

Also: removed unused importInternalLink import (was unused after
InternalLinkPicker replaced the manual modal).

frontend/src/lib/components/notes/MarkdownPreview.svelte

@@ -1,9 +1,12 @@
 <script>
+  import { createEventDispatcher } from 'svelte';
   import { renderMarkdown, isInternalLink, parseVerstakUrl, isAllowedVerstakType } from '../../markdown/markdown';
   import { t } from '../../i18n';
 
   export let content = '';
 
+  const dispatch = createEventDispatcher();
+
   let html = '';
   let error = '';
 
@@ -30,10 +33,7 @@
     const parsed = parseVerstakUrl(href);
     if (!parsed || !isAllowedVerstakType(parsed.type)) return;
 
-    link.dispatchEvent(new CustomEvent('verstak-link', {
-      bubbles: true,
-      detail: { type: parsed.type, id: parsed.id, href },
-    }));
+    dispatch('verstak-link', { type: parsed.type, id: parsed.id, href });
   }
 </script>