Add share URL builders and embed format helpers for uploaded images.

Centralize clearnet/onion paths, delete links, and BBCode/Markdown/HTML output with unit tests and escape-html for safe HTML snippets.

frontend/src/utils/share/buildDeletePath.spec.ts

@@ -0,0 +1,8 @@
+import { describe, expect, it } from 'vitest';
+import { buildDeletePath } from '@/utils/share/buildDeletePath';
+
+describe('buildDeletePath', () => {
+    it('builds delete route with encoded token', () => {
+        expect(buildDeletePath('upload-id', 'abc+def/token')).toBe('/d/upload-id?token=abc%2Bdef%2Ftoken');
+    });
+});