feat(02-02): create AppFooter, default layout, update app.vue with useLocaleHead

- AppFooter with copyright + Gitea/LinkedIn/Fiverr social icons (rel=noopener noreferrer)
- Default layout wraps header + slot + footer with min-h-screen flex
- app.vue uses NuxtLayout + useLocaleHead for global hreflang/canonical
- Fixed a11y.github -> a11y.gitea in both locale files

app/locales/en.json

@@ -28,7 +28,7 @@
     "langToggle": "Change language \u2014 currently English",
     "themeDark": "Switch to light mode",
     "themeLight": "Switch to dark mode",
-    "github": "Killian Dalcin on GitHub (opens in new tab)",
+    "gitea": "Killian Dalcin on Gitea (opens in new tab)",
     "linkedin": "Killian Dalcin on LinkedIn (opens in new tab)",
     "fiverr": "Killian Dalcin on Fiverr (opens in new tab)"
   },

app/locales/fr.json

@@ -28,7 +28,7 @@
     "langToggle": "Changer la langue \u2014 actuellement Francais",
     "themeDark": "Activer le mode clair",
     "themeLight": "Activer le mode sombre",
-    "github": "GitHub de Killian Dalcin (nouvelle fenetre)",
+    "gitea": "Gitea de Killian Dalcin (nouvelle fenetre)",
     "linkedin": "LinkedIn de Killian Dalcin (nouvelle fenetre)",
     "fiverr": "Fiverr de Killian Dalcin (nouvelle fenetre)"
   },