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/components/ConverterCard.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
   import type { Unit, ResultItem } from '$lib/types';
+  import type { Big } from 'big.js';
   import { MIN_COLUMN_WIDTH, GRID_GAP } from '$lib/config';
   import UnitInput from './UnitInput.svelte';
   import UnitDropdown from './UnitDropdown.svelte';
@@ -19,7 +20,8 @@
     oninputchange,
     onunitchange,
     onhighlight,
-    onsetinput
+    onsetinput,
+    onctrlclick
   }: {
     units: Unit[];
     inputValue: number;
@@ -30,6 +32,7 @@
     onunitchange: (id: string) => void;
     onhighlight: (id: string) => void;
     onsetinput: (id: string) => void;
+    onctrlclick: (id: string, value: Big) => void;
   } = $props();
 
   const gridStyle = `--min-col-width: ${MIN_COLUMN_WIDTH}; --grid-gap: ${GRID_GAP};`;
@@ -50,11 +53,12 @@
         isInputUnit={item.unit.id === fromUnitId}
         onclick={() => onhighlight(item.unit.id)}
         onshiftclick={() => onsetinput(item.unit.id)}
+        onctrlclick={() => onctrlclick(item.unit.id, item.convertedValue)}
       />
     {/each}
   </div>
 
   <p class="hotkey-hint">
-    <small>Click a result to highlight it · Shift+click to use it as input</small>
+    <small>Click to highlight · Shift+click to change input unit · Ctrl+click to adopt value as input</small>
   </p>
 </article>