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/admin/validate-file-fragment.php

@@ -0,0 +1,12 @@
+<?php
+/**
+ * validate-file-fragment.php (admin)
+ *
+ * Admin-gated HTMX fragment: validates uploaded file MIME type and size.
+ * Wraps the shared validation logic.
+ */
+require_once __DIR__ . '/../../bootstrap.php';
+require_once __DIR__ . '/../../src/AdminAuth.php';
+AdminAuth::requireLogin();
+
+require_once APP_ROOT . '/src/Controllers/validate-file-fragment-shared.php';