sunset theme following

frontend/src/lib/i18n.ts

@@ -35,6 +35,8 @@ const en = {
   "common.theme": "Theme",
   "common.themeAuto": "Auto",
   "common.themeAutoFull": "Auto (follows system)",
+  "common.themeSunset": "Sunset",
+  "common.themeSunsetFull": "Sunset (follows local time)",
   "common.light": "Light",
   "common.dark": "Dark",
   "common.saveFailed": "Save failed",
@@ -585,6 +587,8 @@ const es: Dict = {
   "common.theme": "Tema",
   "common.themeAuto": "Automático",
   "common.themeAutoFull": "Automático (según el sistema)",
+  "common.themeSunset": "Puesta de sol",
+  "common.themeSunsetFull": "Puesta de sol (hora local)",
   "common.light": "Claro",
   "common.dark": "Oscuro",
   "common.saveFailed": "No se pudo guardar",
@@ -1125,6 +1129,8 @@ const fr: Dict = {
   "common.theme": "Thème",
   "common.themeAuto": "Auto",
   "common.themeAutoFull": "Auto (suit le système)",
+  "common.themeSunset": "Coucher de soleil",
+  "common.themeSunsetFull": "Coucher de soleil (heure locale)",
   "common.light": "Clair",
   "common.dark": "Sombre",
   "common.saveFailed": "Échec de l'enregistrement",
@@ -1665,6 +1671,8 @@ const ru: Dict = {
   "common.theme": "Тема",
   "common.themeAuto": "Авто",
   "common.themeAutoFull": "Авто (как в системе)",
+  "common.themeSunset": "Закат",
+  "common.themeSunsetFull": "Закат (местное время)",
   "common.light": "Светлая",
   "common.dark": "Тёмная",
   "common.saveFailed": "Не удалось сохранить",
@@ -2192,6 +2200,8 @@ const it: Dict = {
   "common.theme": "Tema",
   "common.themeAuto": "Auto",
   "common.themeAutoFull": "Auto (segue il sistema)",
+  "common.themeSunset": "Tramonto",
+  "common.themeSunsetFull": "Tramonto (ora locale)",
   "common.light": "Chiaro",
   "common.dark": "Scuro",
   "common.saveFailed": "Salvataggio non riuscito",
@@ -2725,6 +2735,8 @@ const pt: Dict = {
   "common.theme": "Tema",
   "common.themeAuto": "Auto",
   "common.themeAutoFull": "Auto (segue o sistema)",
+  "common.themeSunset": "Pôr do sol",
+  "common.themeSunsetFull": "Pôr do sol (hora local)",
   "common.light": "Claro",
   "common.dark": "Escuro",
   "common.saveFailed": "Falha ao guardar",

frontend/src/lib/solarTheme.ts

@@ -0,0 +1,39 @@
+/**
+ * "Sunset" theme: follows the browser's local time instead of the OS setting.
+ * Day (07:00–19:00 local) -> light, night -> dark. No geolocation needed.
+ */
+
+export const SOLAR_DAY_START_HOUR = 7;
+export const SOLAR_NIGHT_START_HOUR = 19;
+
+/** True when the given local time falls in the night range (dark theme). */
+export function isSolarDark(now: Date = new Date()): boolean {
+  const h = now.getHours();
+  return h < SOLAR_DAY_START_HOUR || h >= SOLAR_NIGHT_START_HOUR;
+}
+
+/** Milliseconds from `now` until the next 07:00 / 19:00 local boundary. */
+export function msUntilNextSolarSwitch(now: Date = new Date()): number {
+  const next = new Date(now);
+  const h = now.getHours();
+  if (h < SOLAR_DAY_START_HOUR) {
+    next.setHours(SOLAR_DAY_START_HOUR, 0, 0, 0);
+  } else if (h < SOLAR_NIGHT_START_HOUR) {
+    next.setHours(SOLAR_NIGHT_START_HOUR, 0, 0, 0);
+  } else {
+    next.setDate(next.getDate() + 1);
+    next.setHours(SOLAR_DAY_START_HOUR, 0, 0, 0);
+  }
+  return Math.max(1000, next.getTime() - now.getTime());
+}
+
+/** Resolve a stored theme value to dark/light. `sunset` uses local time. */
+export function resolveThemeDark(theme: string, now: Date = new Date()): boolean {
+  if (theme === "dark") return true;
+  if (theme === "light") return false;
+  if (theme === "sunset") return isSolarDark(now);
+  if (typeof window !== "undefined" && window.matchMedia) {
+    return window.matchMedia("(prefers-color-scheme: dark)").matches;
+  }
+  return false;
+}