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/assets/css/form.css

@@ -516,6 +516,22 @@
   color: var(--text-tertiary);
 }
 
+/* ── Inline file validation messages (HTMX) ──────────────────────────────── */
+
+.file-validation-msg {
+  margin-top: var(--space-3xs);
+  font-size: var(--step--2);
+  line-height: 1.5;
+}
+
+.fv-ok {
+  color: var(--success, #2d6a4f);
+}
+
+.fv-error {
+  color: var(--danger, #c1121f);
+}
+
 /* ── TFE file upload queue (.tfe-file-queue) ────────────────────────────── */
 
 .admin-files-fieldgroup {