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/auth-ui.js

@@ -1,5 +1,5 @@
 // static/auth-ui.js — navigation link and back-button handlers
-import { showOnly } from './router.js';
+import { showSection } from './nav.js';
 
 // Data-target navigation (e.g., at #links)
 export function initNavLinks() {
@@ -10,7 +10,7 @@ export function initNavLinks() {
     if (!a || !linksContainer.contains(a)) return;
     e.preventDefault();
     const target = a.dataset.target;
-    if (target) showOnly(target);
+    if (target) showSection(target);
     const burger = document.getElementById('burger-toggle');
     if (burger && burger.checked) burger.checked = false;
   });
@@ -22,7 +22,7 @@ export function initBackButtons() {
     btn.addEventListener('click', e => {
       e.preventDefault();
       const target = btn.dataset.back;
-      if (target) showOnly(target);
+      if (target) showSection(target);
     });
   });
 }