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/routes/+page.svelte

@@ -2,7 +2,6 @@
   import { browser } from '$app/environment';
   import { goto } from '$app/navigation';
   import { page } from '$app/state';
-  import { Big } from 'big.js';
   import { florpUnits } from '$lib/units/definitions';
   import { convertById } from '$lib/convert';
   import { FRACTIONAL_DIGITS } from '$lib/config';
@@ -43,13 +42,12 @@
   }
 
   // ── Derived: results for ALL units in definition order ─────────────────────
+  // convert.ts already handles the same-unit identity (from.id === to.id → return value),
+  // so no special-casing needed here for the from-unit.
   let results = $derived<ResultItem[]>(
     units.map((u) => ({
       unit: u,
-      convertedValue:
-        u.id === fromUnitId
-          ? Big(inputValue) // from-unit always equals inputValue
-          : convertById(inputValue, fromUnitId, u.id, units)
+      convertedValue: convertById(inputValue, fromUnitId, u.id, units)
     }))
   );