feat(permissions) : add role-based UI guards and readonly mode for viewers

- Add usePermissions composable (isAdmin, canEdit, canView)
- Password-protected profile login with modal on profiles page
- Disable all form fields for ROLE_VIEWER across edit/create pages
- Show navigation buttons (Modifier/Consulter) for all roles, hide delete for viewers
- Add readonly prop to ModelTypeForm for category pages
- Disable modal fields (sites, constructeurs) for viewers
- Guard /admin routes in middleware

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/pages/pieces-catalog.vue

@@ -171,6 +171,7 @@
                         Modifier
                       </NuxtLink>
                       <button
+                        v-if="canEdit"
                         type="button"
                         class="btn btn-error btn-xs"
                         :disabled="loadingPieces"
@@ -207,6 +208,7 @@ import DocumentThumbnail from '~/components/DocumentThumbnail.vue'
 import Pagination from '~/components/common/Pagination.vue'
 import { isImageDocument, isPdfDocument } from '~/utils/documentPreview'
 
+const { canEdit } = usePermissions()
 const { showError } = useToast()
 const { pieces, total, loadPieces, loading: loadingPiecesRef, deletePiece } = usePieces()
 const { pieceTypes, loadPieceTypes } = usePieceTypes()