Fix: card text contrast, stable grid order, independent highlight/input states, highlight dismiss

- Explicit dark colors on .result-value, .result-label, .result-symbol, .highlighted-value, .highlighted-label
- Include from-unit in results grid (stable definition-order); from-unit tile shows Big(inputValue) and gets is-input-unit dashed-border class
- Shift+click no longer clears highlightedUnitId — fromUnitId and highlightedUnitId are now fully independent states
- Add × dismiss button (aria-label="Remove highlight") to .highlighted-result banner; .highlighted-result gains position:relative

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

src/components/ConverterCard.svelte

@@ -47,6 +47,7 @@
         value={item.convertedValue}
         unit={item.unit}
         highlighted={item.unit.id === highlightedUnitId}
+        isInputUnit={item.unit.id === fromUnitId}
         onclick={() => onhighlight(item.unit.id)}
         onshiftclick={() => onsetinput(item.unit.id)}
       />