fix: render HTML comments properly in activity feed

Changed approach from stripping HTML on backend to rendering HTML with
sanitization on frontend, matching the pattern used in comment-display
component. This preserves HTML formatting (bold, italics, etc.) in
comment previews whilst still protecting against XSS attacks.

Backend changes:
- Reverted stripHtml() method (no longer needed)
- Keep full HTML content in commentPreview field

Frontend changes:
- Import and inject SanitizeService
- Changed from text interpolation to [innerHTML] with sanitization
- Changed <p> to <div> for comment preview container

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

api/src/app/services/activity.service.ts

@@ -224,12 +224,11 @@ export class ActivityService {
         entityTitle = comment.manga.title;
       }
 
-      // Strip HTML tags and get first 100 characters of comment
-      const plainText = this.stripHtml(comment.content);
+      // Get first 100 characters of comment
       const commentPreview =
-        plainText.length > 100
-          ? `${plainText.slice(0, 100)}...`
-          : plainText;
+        comment.content.length > 100
+          ? `${comment.content.slice(0, 100)}...`
+          : comment.content;
 
       return {
         id: `comment-${comment.id}`,
@@ -351,11 +350,4 @@ export class ActivityService {
         return "Unknown Item";
     }
   }
-
-  /**
-   * Strip HTML tags from content for plain text preview.
-   */
-  private stripHtml(html: string): string {
-    return html.replace(/<[^>]*>/g, "").trim();
-  }
 }