fix: req annexes, add HTMX inline file validation (MIME/size)

- Annexes file input now required when 'has_annexes' checkbox is checked
- PHP-side validation: if has_annexes but no files, throw error
- HTMX inline file validation: POSTs to validate-file-fragment on file change
  - Validates MIME type against per-field whitelists (couverture, note_intention,
    tfe, annexes)
  - Validates file size with PDF-specific 100MB limit
  - Supports both single-file and multi-file inputs
  - Returns green ✓ or red ✕ inline validation messages
- Shared validation logic in src/Controllers/validate-file-fragment-shared.php
- Admin wrapper: admin/validate-file-fragment.php (with AdminAuth guard)
- Partage route: /partage/validate-file-fragment (dispatched via index.php)
- CSS: .file-validation-msg, .fv-ok (green), .fv-error (red)
- file-field.php: accepts $fieldName for per-input validation type,
  auto-detects admin/partage validate URL

app/public/partage/fichiers-fragment.php

@@ -197,7 +197,7 @@ $hasAnnexesChecked = !empty($_POST['has_annexes']);
         $label    = 'Annexes :';
         $accept   = '.pdf,.zip,.tar,.gz';
         $hint     = 'PDF ou archives ZIP/TAR. Max 500 MB.';
-        $required = false;
+        $required = !$adminMode;
         $multiple = true;
         include APP_ROOT . '/templates/partials/form/file-field.php';
         ?>

app/public/partage/index.php

@@ -36,6 +36,13 @@ if ($slug === 'format-website-fragment' && $_SERVER['REQUEST_METHOD'] === 'POST'
     exit;
 }
 
+// Special route: /partage/validate-file-fragment (HTMX fragment — file MIME/size validation)
+if ($slug === 'validate-file-fragment' && $_SERVER['REQUEST_METHOD'] === 'POST') {
+    App::boot();
+    require_once __DIR__ . '/../../src/Controllers/validate-file-fragment-shared.php';
+    exit;
+}
+
 // Special route: /partage/fichiers-fragment (HTMX fragment — format-aware fichiers block)
 if ($slug === 'fichiers-fragment' && $_SERVER['REQUEST_METHOD'] === 'POST') {
     App::boot();