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,