Add font size slider. Closes #132.

frontend/src/utils/fontScale.ts

@@ -0,0 +1,53 @@
+export const FONT_SCALE_KEY = 'remoteterm-font-scale';
+export const DEFAULT_FONT_SCALE = 100;
+export const MIN_FONT_SCALE = 25;
+export const MAX_FONT_SCALE = 400;
+export const FONT_SCALE_SLIDER_STEP = 5;
+
+function normalizeFontScale(scale: number): number {
+  if (!Number.isFinite(scale)) {
+    return DEFAULT_FONT_SCALE;
+  }
+
+  const clamped = Math.min(MAX_FONT_SCALE, Math.max(MIN_FONT_SCALE, scale));
+  return Number.parseFloat(clamped.toFixed(2));
+}
+
+export function getSavedFontScale(): number {
+  try {
+    const raw = localStorage.getItem(FONT_SCALE_KEY);
+    if (raw === null) {
+      return DEFAULT_FONT_SCALE;
+    }
+
+    return normalizeFontScale(Number.parseFloat(raw));
+  } catch {
+    return DEFAULT_FONT_SCALE;
+  }
+}
+
+export function applyFontScale(scale: number): number {
+  const normalized = normalizeFontScale(scale);
+
+  if (typeof document !== 'undefined') {
+    document.documentElement.style.fontSize = `${normalized}%`;
+  }
+
+  return normalized;
+}
+
+export function setSavedFontScale(scale: number): number {
+  const normalized = applyFontScale(scale);
+
+  try {
+    if (normalized === DEFAULT_FONT_SCALE) {
+      localStorage.removeItem(FONT_SCALE_KEY);
+    } else {
+      localStorage.setItem(FONT_SCALE_KEY, String(normalized));
+    }
+  } catch {
+    // localStorage may be unavailable
+  }
+
+  return normalized;
+}