Fix theme flash on navigation

Persist last applied theme in session storage to avoid light-mode flash and toggle flicker when navigating between pages (notably player.html). Also revert README preview back to preview.png for now.

README.md

@@ -5,7 +5,7 @@ Little nodejs + mongodb app that I hack together with ChatGPT and Cursor to keep
 
 **Disclaimer:** This was a fun, experimental project built with “AI slop” (Cursor), so it may have security issues. For example, `python_video_server` uses the default token‑in‑URL approach to serve video files, which can leave user tokens in logs or browser history. There are ways to improve this, but at this point I’m blinder than Ray Charles.
 
-![Oscars Pool 2026](img/preview.gif)
+![Oscars Pool 2026](img/preview.png)
 
 ## Features
 

public/js/app_theme.js

@@ -1,6 +1,7 @@
 // Theme toggle with system preference fallback.
 (function () {
   const STORAGE_KEY = 'theme_preference';
+  const LAST_KEY = 'theme_last_applied';
   const THEME_ATTR = 'data-theme';
   const SOURCE_ATTR = 'data-theme-source';
   const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
@@ -17,6 +18,11 @@
   function setTheme(theme, source) {
     document.documentElement.setAttribute(THEME_ATTR, theme);
     document.documentElement.setAttribute(SOURCE_ATTR, source);
+    try {
+      sessionStorage.setItem(LAST_KEY, theme);
+    } catch (_) {
+      // Ignore storage errors.
+    }
     updateToggleUi(theme, source);
   }
 
@@ -56,12 +62,16 @@
       if (!res.ok) return;
       const data = await res.json();
       const pref = normalizeTheme(data?.themePreference);
-      if (pref) {
-        localStorage.setItem(STORAGE_KEY, pref);
-      } else {
-        localStorage.removeItem(STORAGE_KEY);
+      const currentPref = normalizeTheme(localStorage.getItem(STORAGE_KEY));
+      const changed = pref !== currentPref;
+      if (changed) {
+        if (pref) {
+          localStorage.setItem(STORAGE_KEY, pref);
+        } else {
+          localStorage.removeItem(STORAGE_KEY);
+        }
+        applyStoredOrSystem();
       }
-      applyStoredOrSystem();
     } catch (_) {
       // Ignore fetch errors.
     }
@@ -69,8 +79,14 @@
 
   function applyStoredOrSystem() {
     const stored = normalizeTheme(localStorage.getItem(STORAGE_KEY));
-    const theme = stored || getSystemTheme();
-    const source = stored ? 'user' : 'system';
+    let last = null;
+    try {
+      last = normalizeTheme(sessionStorage.getItem(LAST_KEY));
+    } catch (_) {
+      last = null;
+    }
+    const theme = stored || last || getSystemTheme();
+    const source = stored ? 'user' : (last ? 'session' : 'system');
     setTheme(theme, source);
   }