feat: improve accessibility and SEO (100/100 Lighthouse score)

Fixes color contrast, canonical URLs, viewport scaling, semantic lists,

and resolves 404 errors for manifest/imgproxy.

.pa11yci.json

@@ -2,7 +2,7 @@
   "defaults": {
     "standard": "WCAG2AA",
     "runners": ["axe", "htmlcs"],
-    "ignore": ["color-contrast"],
+    "ignore": [],
     "timeout": 50000,
     "wait": 1000,
     "chromeLaunchConfig": {