fix: accessibility (#57)

* fix accesibility issues:
- index.vue: input without label
- about.vue: color contrast doesn't have sufficient contrast ratio on code sample on light theme

* optimize png icons:
pwa-192x192.png: -31.04%
pwa-512x512.png: -14.66%

use https://ezgif.com/

* chore: color

Co-authored-by: Anthony Fu <anthonyfu117@hotmail.com>

src/styles/markdown.postcss

@@ -8,11 +8,11 @@
 html:not(.dark) {
   --prism-foreground: #393a34;
   --prism-background: #fbfbfb;
-  --prism-comment: #b8c4b8;
-  --prism-string: #c67b5d;
+  --prism-comment: #8e8f8e;
+  --prism-string: #a1644c;
   --prism-literal: #3a9c9b;
-  --prism-keyword: #248459;
-  --prism-function: #849145;
+  --prism-keyword: #248358;
+  --prism-function: #7e8a42;
   --prism-deleted: #a14f55;
   --prism-class: #2b91af;
   --prism-builtin: #a52727;