Add the official social marks to the footer links

The marks come from the simple-icons package (CC0) and are inlined at build
time — the earlier hand-drawn glyphs were wrong, the X mark in particular is
not a plain cross. Importing the three SVGs individually rather than the
package index keeps the other icons out of the bundle: 10 kB, not 5 MB.

Rendered monochrome in the surrounding text colour, with the text label kept
next to each one.

frontend/package-lock.json

@@ -10,6 +10,7 @@
         "@nuxtjs/i18n": "^10.6.0",
         "marked": "^18.0.11",
         "nuxt": "^4.5.2",
+        "simple-icons": "^16.28.0",
         "vue": "^3.5.41",
         "vue-router": "^5.2.0"
       },
@@ -10241,6 +10242,25 @@
         "url": "https://github.com/steveukx/git-js?sponsor=1"
       }
     },
+    "node_modules/simple-icons": {
+      "version": "16.28.0",
+      "resolved": "https://registry.npmjs.org/simple-icons/-/simple-icons-16.28.0.tgz",
+      "integrity": "sha512-sQPR5AtK/ijRjou7zw7mlLp08oB6FH7i0lOy5XJ2zp9mJs/yejgiOn7KvQoe2q4YJIx6VmgUSW5AOefebPt5kg==",
+      "funding": [
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/simple-icons"
+        },
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/simple-icons"
+        }
+      ],
+      "license": "CC0-1.0",
+      "engines": {
+        "node": ">=0.12.18"
+      }
+    },
     "node_modules/sirv": {
       "version": "3.0.2",
       "resolved": "https://registry.npmjs.org/sirv/-/sirv-3.0.2.tgz",