Admin UX: light/dark theme, gallery pagination, content modal + delete, openai type

- Light/dark theme toggle persisted in localStorage
- Gallery pagination: page size 50/100/150/200 with prev/next
- Fullscreen modal viewer (image/video/audio) with metadata and delete
- Delete content: DELETE /api/generations/:id removes file (media-dir guarded)
  and journal record; added Journal.deleteById
- Provider types now include openai
- HTML output escaping in UI

NexusAI/packages/core/src/db/journal.ts

@@ -106,6 +106,11 @@ export class Journal {
     return rows as GenerationRecord[];
   }
 
+  deleteById(id: string): boolean {
+    const r = this.db.prepare('DELETE FROM generations WHERE id = ?').run(id);
+    return r.changes > 0;
+  }
+
   stats(sinceDays?: number): UsageStats {
     const filter = sinceDays
       ? `WHERE created_at >= datetime('now', '-${Math.max(1, Math.floor(sinceDays))} days')`