Fix: shift+click preserves input value, Ctrl+click adopts value, stable highlight layout

- shift+click now only sets fromUnitId; inputValue is unchanged so all outputs
  recalculate correctly from the original user input
- Ctrl+click is a new gesture that sets both fromUnitId and inputValue to the
  tile's displayed converted value (the "pivot" action)
- highlighted-result block always renders with min-height:8rem so the page
  below it never jumps when a highlight is added or cleared
- hotkey hint updated to document all three click gestures

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

src/app.css

@@ -97,6 +97,7 @@
   align-items: center;
   gap: 0.4rem;
   position: relative;
+  min-height: 8rem;
 }
 
 .highlighted-value {