feat(theme,ui): apply pure white background in light mode and improve accessibility

- Update light mode theme tokens to pure white (#ffffff) background and soft gray borders
- Add supports-[height:100dvh] fallback to ProfessorDetailsDialog max-height
- Add aria-hidden to decorative arrows in Header mobile navigation

refer_landing_page/app/globals.css

@@ -41,10 +41,10 @@
 
   --ink-rgb: 10 10 10;
   --ink-soft-rgb: 38 36 32;
-  --ink-mute-rgb: 107 101 91;
-  --ivory-rgb: 245 241 232;
-  --paper-rgb: 236 230 214;
-  --line-rgb: 216 209 192;
+  --ink-mute-rgb: 107 114 128;
+  --ivory-rgb: 255 255 255;
+  --paper-rgb: 248 249 250;
+  --line-rgb: 229 231 235;
   --vermillion-rgb: 217 52 43;
   --vermillion-dark-rgb: 168 35 28;
   --cobalt-rgb: 78 119 255;
@@ -61,7 +61,7 @@
   --cobalt: rgb(var(--cobalt-rgb));
   --cobalt-dark: rgb(var(--cobalt-dark-rgb));
 
-  --grain-color: rgba(10, 10, 10, 0.022);
+  --grain-color: rgba(10, 10, 10, 0.015);
 
   /* CUSTOMIZATION HOOK — MOTION: global reveal timing */
   --reveal-duration: 600ms;