feat: Overhaul client-side navigation and clean up project

- Implement a unified SPA routing system in nav.js, removing all legacy and conflicting navigation scripts (router.js, inject-nav.js, fix-nav.js).
- Refactor dashboard.js to delegate all navigation handling to the new nav.js module.
- Create new modular JS files (auth.js, personal-player.js, logger.js) to improve code organization.
- Fix all navigation-related bugs, including guest access and broken footer links.
- Clean up the project root by moving development scripts and backups to a dedicated /dev directory.
- Add a .gitignore file to exclude the database, logs, and other transient files from the repository.

static/logger.js

@@ -0,0 +1,6 @@
+export function logToServer(msg) {
+  const xhr = new XMLHttpRequest();
+  xhr.open("POST", "/log", true);
+  xhr.setRequestHeader("Content-Type", "application/json");
+  xhr.send(JSON.stringify({ msg }));
+}