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/package.json

@@ -16,6 +16,7 @@
         "altcha-lib": "^2.7.0",
         "axios": "^1.20.0",
         "dayjs": "^1.11.23",
+        "escape-html": "^1.0.3",
         "hash-wasm": "^4.12.0",
         "pinia": "^4.0.3",
         "pretty-bytes": "^7.2.0",
@@ -24,6 +25,7 @@
         "vue-router": "^4.6.4"
     },
     "devDependencies": {
+        "@types/escape-html": "^1.0.4",
         "@types/node": "^22.10.7",
         "@vitejs/plugin-vue": "^6.0.5",
         "@vue/tsconfig": "^0.9.1",