Simplify: promote inline Big import type, merge duplicate hover/highlighted CSS rules

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/routes/+page.svelte

@@ -6,6 +6,7 @@
   import { convertById } from '$lib/convert';
   import { FRACTIONAL_DIGITS } from '$lib/config';
   import { formatBig } from '$lib/format';
+  import type { Big } from 'big.js';
   import type { Unit, ResultItem } from '$lib/types';
   import ConverterCard from '../components/ConverterCard.svelte';
 
@@ -94,7 +95,7 @@
     fromUnitId = id;
   }
 
-  function handleCtrlClick(id: string, convertedValue: import('big.js').Big) {
+  function handleCtrlClick(id: string, convertedValue: Big) {
     // Ctrl+click: adopt this tile's displayed value as the new inputValue and promote it to from-unit.
     // This is the "pivot" gesture.
     inputValue = parseFloat(convertedValue.toFixed(FRACTIONAL_DIGITS));