Simplify: PHP page dedup, secure UUID, input validation, remove redundant headers

- pages/*.php: 11 files reduced to 1-liners via shared renderPage() helper
- Session: UUID generation uses random_bytes() instead of mt_rand()
- Session: logout cookie uses same security options as create
- API saves: size limits on key (100) and data (500KB)
- API sessions: displayName capped at 64 chars
- API: removed redundant Content-Type headers (Response::json handles it)
- API dashboard: replaced SELECT * with explicit columns

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

App/php/api/assessment.php

@@ -4,8 +4,6 @@
  * POST /api/assessment {simId, processLog, predictions, results, reflections, duration, completedPhases}
  */
 
-header('Content-Type: application/json; charset=utf-8');
-
 if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
     Response::error('Nur POST erlaubt', 405);
 }