feat: rename memoire to tfe and improve styling

- Rename memoire.php to tfe.php throughout codebase
- Create dedicated tfe.css with rounded header/main/footer layout
- Move metadata (orientation, AP program, finality, keywords) to header
- Move back button from header to footer
- Create shared templates/head.php for common HTML head section
- Maintain rounded borders (40px) matching main site design
- Keep purple header (#9557b5), green main (#3c856b), dark footer (#222)
- Improve content readability with centered max-width layout
- Add responsive design for mobile devices

public/assets/main.css

@@ -11,12 +11,12 @@ body {
 
 /* Layout ratios: header 2/5, main 3/5, footer 1/5 */
 header {
-  flex: 1;
+  flex: 2;
   min-height: 0;
 }
 
 main {
-  flex: 4;
+  flex: 7;
   min-height: 0;
 }