Default to the dark theme for a fresh visitor

With no stored preference the app now resolves to dark instead of
tracking the OS. An explicit choice still wins: light/dark persist as
before, and "system" is now persisted verbatim rather than as a
cleared key, so an explicit OS-tracking choice stays distinguishable
from a never-chosen state and survives a reload. The first-paint
bootstrap script mirrors the same resolution so a fresh visitor sees
no flash to light.

src/app/layout.tsx

@@ -90,7 +90,7 @@ export const viewport: Viewport = {
 };
 
 // Inline script to apply theme before first paint (prevents FOUC)
-const themeScript = `(function(){try{var t=localStorage.getItem("healthlog-theme");var c=(t==="light"||t==="dark")?t:(window.matchMedia("(prefers-color-scheme:dark)").matches?"dark":"light");document.documentElement.classList.add(c)}catch(e){document.documentElement.classList.add("dark")}})()`;
+const themeScript = `(function(){try{var t=localStorage.getItem("healthlog-theme");var c=(t==="light"||t==="dark")?t:(t==="system"?(window.matchMedia("(prefers-color-scheme:dark)").matches?"dark":"light"):"dark");document.documentElement.classList.add(c)}catch(e){document.documentElement.classList.add("dark")}})()`;
 
 export default async function RootLayout({
   children,

src/components/providers.tsx

@@ -49,16 +49,21 @@ function applyTheme(resolved: "dark" | "light") {
 
 function ThemeProvider({ children }: { children: ReactNode }) {
   const [theme, setThemeState] = useState<Theme>(() => {
-    if (typeof window === "undefined") return "system";
+    if (typeof window === "undefined") return "dark";
     const saved = localStorage.getItem("healthlog-theme") as Theme | null;
-    return saved === "light" || saved === "dark" ? saved : "system";
+    // A stored "light"/"dark"/"system" choice wins. With no stored
+    // preference the app defaults to dark rather than tracking the OS.
+    if (saved === "light" || saved === "dark") return saved;
+    if (saved === "system") return "system";
+    return "dark";
   });
 
   const [resolvedTheme, setResolvedTheme] = useState<"dark" | "light">(() => {
     if (typeof window === "undefined") return "dark";
     const saved = localStorage.getItem("healthlog-theme") as Theme | null;
-    const t = saved === "light" || saved === "dark" ? saved : "system";
-    return t === "system" ? getSystemTheme() : t;
+    if (saved === "light" || saved === "dark") return saved;
+    if (saved === "system") return getSystemTheme();
+    return "dark";
   });
 
   // Apply theme on mount (inline script handles FOUC, this ensures classes are in sync)
@@ -82,13 +87,16 @@ function ThemeProvider({ children }: { children: ReactNode }) {
 
   const setTheme = useCallback((next: Theme) => {
     setThemeState(next);
+    // Persist every explicit choice, including "system" — the absence of a
+    // stored value is reserved for a fresh visitor and now defaults to dark.
+    // Storing "system" verbatim keeps an explicit OS-tracking choice
+    // distinguishable from "never chose", so a reload honours it.
+    localStorage.setItem("healthlog-theme", next);
     if (next === "system") {
-      localStorage.removeItem("healthlog-theme");
       const resolved = getSystemTheme();
       setResolvedTheme(resolved);
       applyTheme(resolved);
     } else {
-      localStorage.setItem("healthlog-theme", next);
       setResolvedTheme(next);
       applyTheme(next);
     }