feat: add algorithmic size and font scales

Add :scales config to tokens.edn with two generated scales:
- Size (linear): --size-1 through --size-16, base 0.25rem × N
- Font (geometric): --font-xs through --font-3xl, ratio 1.25

gen.clj computes values and emits them into :root only (not dark
theme blocks). button.css now uses scale vars instead of raw rem.
Change base or ratio in tokens.edn to recompute the entire scale.

src/ui/button.css

@@ -3,10 +3,10 @@
   align-items: center;
   justify-content: center;
   gap: 0.5em;
-  padding: 0.5rem 1rem;
-  font-size: 0.875rem;
+  padding: var(--size-2) var(--size-4);
+  font-size: var(--font-sm);
   font-weight: 500;
-  line-height: 1.25rem;
+  line-height: var(--size-5);
   border: none;
   border-radius: var(--radius-md);
   cursor: pointer;
@@ -48,15 +48,15 @@
 }
 
 .btn-sm {
-  padding: 0.25rem 0.5rem;
-  font-size: 0.75rem;
-  line-height: 1rem;
+  padding: var(--size-1) var(--size-2);
+  font-size: var(--font-xs);
+  line-height: var(--size-4);
 }
 
 .btn-lg {
-  padding: 0.75rem 1.5rem;
-  font-size: 1rem;
-  line-height: 1.5rem;
+  padding: var(--size-3) var(--size-6);
+  font-size: var(--font-base);
+  line-height: var(--size-6);
 }
 
 .btn:disabled {