feat(theme): add HSL-based color scale generation with jon/color-tools

Replace hardcoded hex color tokens with algorithmic color scales generated
from HSL parameters. Each scale is defined by hue, saturation, and
lightness steps in tokens.edn, then converted to hex via jon/color-tools.

Color scales (gray, accent, danger, success, warning) generate 11 stops
each (50–950) into :root. Semantic tokens (bg-0, fg-0, accent, etc.)
reference scale variables with var(--gray-50), var(--accent-500), etc.
Dark theme switches which stop each semantic token points to.

Gray scale uses hue 240 with tapered saturation for a purplish tint
matching the activity-tracker aesthetic. Accent is vivid purple (hue 252).
Border radii bumped to 6/10/16px for a rounder feel.

To shift the entire palette (e.g. warm gray), change hue/saturation in
tokens.edn and run `bb build-theme`.

bb.edn

@@ -1,4 +1,5 @@
-{:paths ["src" "test" "dev/hiccup/src"]
+{:deps {com.github.jramosg/color-tools {:mvn/version "1.1.0"}}
+ :paths ["src" "test" "dev/hiccup/src"]
 
  :tasks
  {build-theme