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/routes/+page.svelte

@@ -89,9 +89,14 @@
 
   function handleSetInput(id: string) {
     // Shift+click: promote result unit to from-unit.
-    // Re-express the current value in terms of the new from-unit.
+    // inputValue is NOT changed — outputs recalculate from the same inputValue with the new fromUnitId.
     // highlightedUnitId is NOT cleared — the two states are independent.
-    const convertedValue = convertById(inputValue, fromUnitId, id, units);
+    fromUnitId = id;
+  }
+
+  function handleCtrlClick(id: string, convertedValue: import('big.js').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));
     fromUnitId = id;
   }
@@ -101,8 +106,8 @@
   <title>Humor Units — Converter</title>
 </svelte:head>
 
-{#if highlightedResult !== null}
-  <div class="highlighted-result">
+<div class="highlighted-result" aria-live="polite">
+  {#if highlightedResult !== null}
     <button
       class="highlight-dismiss"
       aria-label="Remove highlight"
@@ -111,8 +116,8 @@
     <span class="highlighted-value">{formatBig(highlightedResult.convertedValue)}</span>
     <span class="highlighted-label">{highlightedResult.unit.labelPlural}</span>
     <span class="highlighted-symbol">{highlightedResult.unit.symbol}</span>
-  </div>
-{/if}
+  {/if}
+</div>
 
 <ConverterCard
   {units}
@@ -124,4 +129,5 @@
   onunitchange={handleUnitChange}
   onhighlight={handleHighlight}
   onsetinput={handleSetInput}
+  onctrlclick={handleCtrlClick}
 />