New Aero background (Gemini-made), old credit removed

The scene is the user's own Frutiger Aero image now: green hills, a river,
an eco-city, bubbles and daisies. Re-encoded to public/aero/background.webp
(152 kB) with a fresh 3 kB pre-blurred variant for the glass, and the
WallpaperSafari credit line, its i18n keys and the README credit are gone
(the image is made by the user with Gemini).

Contrast: 0 below-minimum findings and 0 axe violations, all three themes.
Version 0.2.8, SW -> freezmo-v60.

lib/theme.ts

@@ -0,0 +1,92 @@
+"use client";
+
+import { useSyncExternalStore } from "react";
+
+/**
+ * The theme, as a shared store. It used to be component state inside
+ * `useTheme()`, which meant every caller (header, rail, chat, Settings) had
+ * its own copy: switching to Aero in Settings left the rail's light/dark row
+ * on screen until a reload. One store, one snapshot, everyone re-renders.
+ */
+
+const STORAGE_KEY = "freezmo.theme";
+
+const THEME_COLORS = { light: "#f4f7fd", dark: "#0e1424", aero: "#2fa7e6" } as const;
+
+/** The three themes. Aero is not a dark/light variant: it replaces the palette
+ *  and the scene, and the light/dark toggles hide while it is on. */
+export type Theme = "light" | "dark" | "aero";
+
+let cached: Theme | null = null;
+const listeners = new Set<() => void>();
+
+function read(): Theme {
+  try {
+    const stored = window.localStorage.getItem(STORAGE_KEY);
+    if (stored === "light" || stored === "dark" || stored === "aero") return stored;
+    return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+  } catch {
+    /* private mode */
+    return "light";
+  }
+}
+
+function snapshot(): Theme {
+  cached ??= read();
+  return cached;
+}
+
+function serverSnapshot(): Theme {
+  return "light";
+}
+
+/**
+ * Keeps the first `theme-color` meta (the one the app owns) in step with the
+ * in-app theme. The SSR metas follow the OS preference; this one always wins
+ * because it is media-less and first in the head.
+ */
+export function syncThemeColor(theme: Theme) {
+  try {
+    let meta = document.querySelector<HTMLMetaElement>('meta[name="theme-color"][data-app]');
+    if (!meta) {
+      meta = document.createElement("meta");
+      meta.setAttribute("name", "theme-color");
+      meta.setAttribute("data-app", "1");
+      document.head.insertBefore(meta, document.head.firstChild);
+    }
+    meta.setAttribute("content", THEME_COLORS[theme]);
+  } catch {
+    /* private mode / odd embedded browsers */
+  }
+}
+
+export function applyTheme(next: Theme) {
+  document.documentElement.classList.toggle("dark", next === "dark");
+  document.documentElement.classList.toggle("aero", next === "aero");
+  syncThemeColor(next);
+  cached = next;
+  try {
+    window.localStorage.setItem(STORAGE_KEY, next);
+  } catch {
+    /* private mode */
+  }
+  listeners.forEach((listener) => listener());
+}
+
+export function useTheme() {
+  const theme = useSyncExternalStore(
+    (listener) => {
+      listeners.add(listener);
+      return () => listeners.delete(listener);
+    },
+    snapshot,
+    serverSnapshot,
+  );
+
+  return {
+    theme,
+    setTheme: applyTheme,
+    /** The light/dark flip; aero is chosen in Settings. */
+    toggle: () => applyTheme(theme === "dark" ? "light" : "dark"),
+  };
+}