Simplify: token-ify hardcoded colors, remove redundant Big ternary, guard no-op shift-click

- Replace 5 hardcoded #2C2C2C/#555550/#888880 literals with var(--pico-color),
  var(--color-label), var(--pico-muted-color); add --color-label token to :root
- Remove inline Big(inputValue) ternary in results derived — convert.ts already
  handles same-unit identity via its own from.id === to.id guard
- Drop now-unused Big import from +page.svelte
- Guard onshiftclick in ConversionResult: no-op when tile is already the from-unit

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

src/components/ConversionResult.svelte

@@ -29,7 +29,9 @@
 
   function handleClick(e: MouseEvent) {
     if (e.shiftKey) {
-      onshiftclick();
+      // Shift+click promotes this tile to the from-unit.
+      // Guard: if it already IS the from-unit, the action would be a no-op.
+      if (!isInputUnit) onshiftclick();
     } else {
       onclick();
     }