feat(themes): dedicated code-syntax palette keys

Code highlighting reused brand tokens (accent/text/border/muted), so it couldn't
be themed independently. Add syntax_string/number/keyword/comment skin keys →
syntax* theme tokens (defaulting to those brand tokens, so defaults are
unchanged) and point the highlighter at them. Documented in the element→key map.

apps/shared/src/skin.ts

@@ -44,6 +44,10 @@ export const SKIN_COLOR_TOKENS = [
   'diff_removed',
   'diff_added_word',
   'diff_removed_word',
+  'syntax_string',
+  'syntax_number',
+  'syntax_keyword',
+  'syntax_comment',
   // CLI / TUI chrome.
   'prompt',
   'input_rule',