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/add.php

@@ -57,7 +57,7 @@ function wasSelected($key, $value)
     return $formData[$key] == $value;
 }
 ?>
-<?php require_once __DIR__ . "/inc/head.php"; ?>
+<?php require_once APP_ROOT . '/templates/admin/head.php'; ?>
 <main>
     <?php if ($error): ?>
         <div class="error-message">
@@ -260,4 +260,4 @@ function wasSelected($key, $value)
     </form>
 </main>
 
-<?php require_once __DIR__ . "/inc/footer.php"; ?>
+<?php require_once APP_ROOT . '/templates/admin/footer.php'; ?>