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/app.css

@@ -11,6 +11,7 @@
   --pico-card-border-color: #E8E4DF;
   --pico-color: #2C2C2C;
   --pico-muted-color: #888880;
+  --color-label: #555550; /* mid-muted: darker than muted-color, lighter than body text */
   --pico-muted-border-color: #E0DCD7;
   --pico-primary: #5B7FA6;
   --pico-primary-background: rgba(91, 127, 166, 0.08);
@@ -68,17 +69,17 @@
   font-weight: 600;
   font-variant-numeric: tabular-nums;
   word-break: break-all;
-  color: #2C2C2C; /* explicit dark charcoal, do not rely on inheritance */
+  color: var(--pico-color); /* explicit; do not rely on inheritance */
 }
 
 .result-label {
   font-size: 0.85rem;
-  color: #555550;
+  color: var(--color-label);
 }
 
 .result-symbol {
   font-size: 0.75rem;
-  color: #888880;
+  color: var(--pico-muted-color);
   font-style: italic;
 }
 
@@ -104,12 +105,12 @@
   font-variant-numeric: tabular-nums;
   line-height: 1.1;
   word-break: break-all;
-  color: #2C2C2C;
+  color: var(--pico-color);
 }
 
 .highlighted-label {
   font-size: 1.25rem;
-  color: #2C2C2C;
+  color: var(--pico-color);
 }
 
 .highlighted-symbol {
@@ -160,7 +161,7 @@
 }
 
 .highlight-dismiss:hover {
-  color: #2C2C2C;
+  color: var(--pico-color);
   background-color: rgba(0, 0, 0, 0.06);
 }