fix: scoped HTMX file validation, add validation to TFE/PeerTube inputs

- Wrap file-field.php validation in <form> to scope hx-include (fixes
  cross-field contamination where cover change triggered note_intention
  validation)
- Add inline MIME/size validation to upload-tfe-file.php
- Add inline validation to PeerTube video/audio and direct video/audio
  file inputs in format-extras-block
- Fallback in validate-file-fragment-shared.php: if field_name doesn't
  match any $_FILES key, try the first uploaded file (handles
  PeerTube inputs where name differs from field_name)
- Fix file-field.php admin_mode using $adminMode variable instead of
  undefined ADMIN_MODE constant

app/public/.htaccess

@@ -1,16 +1,20 @@
 # PHP upload limits for large thesis files (PDFs, video, audio)
+# TFE files are uploaded incrementally via HTMX (one at a time).
+# PeerTube/direct video/audio uploads + couverture + note_intention
+# are submitted in the final form POST — post_max_size must accommodate
+# the largest combination.
 <IfModule mod_php.c>
     php_value upload_max_filesize 512M
-    php_value post_max_size       520M
-    php_value memory_limit        256M
-    php_value max_execution_time  300
+    php_value post_max_size       1024M
+    php_value memory_limit        512M
+    php_value max_execution_time  600
 </IfModule>
 # mod_php8 variant
 <IfModule mod_php8.c>
     php_value upload_max_filesize 512M
-    php_value post_max_size       520M
-    php_value memory_limit        256M
-    php_value max_execution_time  300
+    php_value post_max_size       1024M
+    php_value memory_limit        512M
+    php_value max_execution_time  600
 </IfModule>
 
 # Prevent directory listing