Add local label

frontend/src/utils/localLabel.ts

@@ -0,0 +1,48 @@
+const LOCAL_LABEL_KEY = 'remoteterm-local-label';
+
+export interface LocalLabel {
+  text: string;
+  color: string;
+}
+
+const DEFAULT_LABEL: LocalLabel = { text: '', color: '#062d60' };
+
+export function getLocalLabel(): LocalLabel {
+  try {
+    const raw = localStorage.getItem(LOCAL_LABEL_KEY);
+    if (!raw) return DEFAULT_LABEL;
+    const parsed = JSON.parse(raw) as Partial<LocalLabel>;
+    return {
+      text: typeof parsed.text === 'string' ? parsed.text : '',
+      color: typeof parsed.color === 'string' ? parsed.color : DEFAULT_LABEL.color,
+    };
+  } catch {
+    return DEFAULT_LABEL;
+  }
+}
+
+export function setLocalLabel(text: string, color: string): void {
+  try {
+    localStorage.setItem(LOCAL_LABEL_KEY, JSON.stringify({ text, color }));
+  } catch {
+    // localStorage may be unavailable
+  }
+}
+
+/**
+ * Returns 'white' or 'black' for best contrast against the given hex color.
+ * Uses WCAG relative luminance formula.
+ */
+export function getContrastTextColor(hexColor: string): string {
+  const hex = hexColor.replace('#', '');
+  const r = parseInt(hex.substring(0, 2), 16) / 255;
+  const g = parseInt(hex.substring(2, 4), 16) / 255;
+  const b = parseInt(hex.substring(4, 6), 16) / 255;
+
+  // sRGB to linear
+  const toLinear = (c: number) => (c <= 0.03928 ? c / 12.92 : ((c + 0.055) / 1.055) ** 2.4);
+  const luminance = 0.2126 * toLinear(r) + 0.7152 * toLinear(g) + 0.0722 * toLinear(b);
+
+  // WCAG threshold: luminance > 0.179 → dark text, else light text
+  return luminance > 0.179 ? 'black' : 'white';
+}