feat(subtitle-sidebar): add sidebar config surface (#28)

src/config/resolve/shared.ts

@@ -15,6 +15,22 @@ export function asBoolean(value: unknown): boolean | undefined {
 }
 
 const hexColorPattern = /^#(?:[0-9a-fA-F]{3}|[0-9a-fA-F]{4}|[0-9a-fA-F]{6}|[0-9a-fA-F]{8})$/;
+const cssColorKeywords = new Set([
+  'transparent',
+  'currentcolor',
+  'inherit',
+  'initial',
+  'unset',
+  'revert',
+  'revert-layer',
+]);
+const cssColorFunctionPattern = /^(?:rgba?|hsla?)\(\s*[^()]+?\s*\)$/i;
+
+function supportsCssColor(text: string): boolean {
+  const css = (globalThis as { CSS?: { supports?: (property: string, value: string) => boolean } })
+    .CSS;
+  return css?.supports?.('color', text) ?? false;
+}
 
 export function asColor(value: unknown): string | undefined {
   if (typeof value !== 'string') return undefined;
@@ -22,6 +38,30 @@ export function asColor(value: unknown): string | undefined {
   return hexColorPattern.test(text) ? text : undefined;
 }
 
+export function asCssColor(value: unknown): string | undefined {
+  if (typeof value !== 'string') return undefined;
+
+  const text = value.trim();
+  if (text.length === 0) {
+    return undefined;
+  }
+
+  if (supportsCssColor(text)) {
+    return text;
+  }
+
+  const normalized = text.toLowerCase();
+  if (
+    hexColorPattern.test(text) ||
+    cssColorKeywords.has(normalized) ||
+    cssColorFunctionPattern.test(text)
+  ) {
+    return text;
+  }
+
+  return undefined;
+}
+
 export function asFrequencyBandedColors(
   value: unknown,
 ): [string, string, string, string, string] | undefined {