feat(web/m7-362): theme preference + resolved-theme rune store

web/src/lib/stores/theme.svelte.ts

@@ -0,0 +1,62 @@
+const STORAGE_KEY = 'minstrel:theme';
+
+export type ThemePreference = 'dark' | 'light' | 'system';
+export type ResolvedTheme = 'dark' | 'light';
+
+function readPref(): ThemePreference {
+  if (typeof localStorage === 'undefined') return 'system';
+  try {
+    const v = localStorage.getItem(STORAGE_KEY);
+    return v === 'dark' || v === 'light' || v === 'system' ? v : 'system';
+  } catch {
+    return 'system';
+  }
+}
+
+function systemResolved(): ResolvedTheme {
+  if (typeof window === 'undefined' || !window.matchMedia) return 'dark';
+  return window.matchMedia('(prefers-color-scheme: light)').matches ? 'light' : 'dark';
+}
+
+function resolve(pref: ThemePreference): ResolvedTheme {
+  return pref === 'system' ? systemResolved() : pref;
+}
+
+function applyToDOM(resolved: ResolvedTheme): void {
+  if (typeof document === 'undefined') return;
+  document.documentElement.setAttribute('data-theme', resolved);
+}
+
+let _theme = $state<ThemePreference>(readPref());
+let _resolved = $state<ResolvedTheme>(resolve(_theme));
+
+export const theme = {
+  get value(): ThemePreference {
+    return _theme;
+  }
+};
+
+export const resolvedTheme = {
+  get value(): ResolvedTheme {
+    return _resolved;
+  }
+};
+
+export function setTheme(pref: ThemePreference): void {
+  _theme = pref;
+  _resolved = resolve(pref);
+  if (typeof localStorage !== 'undefined') {
+    try { localStorage.setItem(STORAGE_KEY, pref); } catch { /* ignore quota / unavailable */ }
+  }
+  applyToDOM(_resolved);
+}
+
+if (typeof window !== 'undefined' && window.matchMedia) {
+  const mq = window.matchMedia('(prefers-color-scheme: light)');
+  mq.addEventListener('change', () => {
+    if (_theme === 'system') {
+      _resolved = systemResolved();
+      applyToDOM(_resolved);
+    }
+  });
+}