refactor: extract templates from public/

- Created /templates for main site (header.php, footer.php)
- Created /templates/admin for admin section (head.php, footer.php)
- Removed /public/includes and /public/admin/inc
- Updated all references in code and docs
- Tests passing ✅

Cleaner separation: /public only contains web-accessible files (PHP entry points + assets)

public/admin/thanks.php

@@ -71,7 +71,7 @@ function formatFileSize($bytes) {
 // Set page title for header
 $pageTitle = "Merci";
 ?>
-<?php require_once __DIR__ . '/inc/head.php'; ?>
+<?php require_once APP_ROOT . '/templates/admin/head.php'; ?>
 
     <main>
         <?php if ($error): ?>
@@ -198,4 +198,4 @@ $pageTitle = "Merci";
         <?php endif; ?>
     </main>
 
-<?php require_once __DIR__ . '/inc/footer.php'; ?>
+<?php require_once APP_ROOT . '/templates/admin/footer.php'; ?>