feat(frontend): add light/dark theme switcher

Add persisted theme mode (light, dark, system) with a header menu.
Rework the light and dark palettes for stronger contrast and drop the
high-contrast theme.

client/src/styles/a11y.css

@@ -1,12 +1,9 @@
 .v-theme--light {
-  --a11y-focus: #0b3d91;
+  --a11y-focus: #4c1d95;
 }
 .v-theme--dark {
   --a11y-focus: #ffd54f;
 }
-.v-theme--highContrast {
-  --a11y-focus: #ffff00;
-}
 
 html {
   font-size: 112.5%;