Add lightweight self-hosted Hugo CMS with GitHub API backend

Implements a Go-based CMS for editing Hugo posts and uploading images
via the GitHub REST API, with no database and Git as the source of truth.

Features:
- Single admin auth with bcrypt password and signed session cookies
- Dashboard, post listing with search, create/edit with EasyMDE editor
- Media library for static/uploads/ with drag-drop and clipboard paste
- Client-side autosave, unsaved changes warning, dark mode, responsive UI
- Modular packages for auth, github, posts, media, session, and handlers

Co-authored-by: codegirl007 <s.raide@gmail.com>

cms/internal/templates/static/js/app.js

@@ -0,0 +1,24 @@
+(function () {
+  'use strict';
+
+  const THEME_KEY = 'cms-theme';
+
+  function initTheme() {
+    const saved = localStorage.getItem(THEME_KEY);
+    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
+    const theme = saved || (prefersDark ? 'dark' : 'light');
+    document.documentElement.setAttribute('data-theme', theme);
+
+    const toggle = document.getElementById('theme-toggle');
+    if (toggle) {
+      toggle.addEventListener('click', function () {
+        const current = document.documentElement.getAttribute('data-theme');
+        const next = current === 'dark' ? 'light' : 'dark';
+        document.documentElement.setAttribute('data-theme', next);
+        localStorage.setItem(THEME_KEY, next);
+      });
+    }
+  }
+
+  document.addEventListener('DOMContentLoaded', initTheme);
+})();