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.json

@@ -14,6 +14,7 @@
     "@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"
   },