feat(footer): CMS-editable copyright with 2002–current year range

The copyright text lived in site.ts with a build-time year and was not
CMS-editable. It now lives in common.json (footer.copyright, editable via
the Közös szövegek tab) with a {year} placeholder that resolves to the
current year at render: '© 2002–{year} mozdIT Bt. Minden jog fenntartva.'
Schema and types extended; site.ts keeps only the footer links.

Closes MITHOME-66

proto/src/content/common.json

@@ -16,5 +16,8 @@
   },
   "staging": {
     "banner": "⚠ STAGING / TESZTKÖRNYEZET — A STAGING OLDALT LÁTOD"
+  },
+  "footer": {
+    "copyright": "© 2002–{year} mozdIT Bt. Minden jog fenntartva."
   }
 }