Learn UI: prev/next navigation, sidebar outline, editing on every page; merge import; defense date

- article page: ← / → navigation across the whole material tree (including
  between sections), sticky sidebar outline with the current article highlighted
- editing on chapter, section and article pages: rename, publish/hide, delete,
  create section/article, move articles while deleting a section; forms return
  to the page they were opened from (validated internal `back` parameter)
- article page opens the edit dialog directly via ?edit=1 (used by "Править"
  links on section/chapter pages)
- import: optional sync flag — with it stale sections/articles of archived
  chapters are deleted (previous behaviour), without it import only adds and
  updates, so material edited in the UI is not overwritten
- student area: lab page and lab cards now show the defense date next to the
  submission deadline
- fix SpEL crash on the student lab page (`and` with nullable strings)

services/academic-service/src/main/kotlin/ru/ysu/lms/academic/content/ContentService.kt

@@ -400,7 +400,7 @@ class ContentService(
      * картинки в assets/.
      */
     @Transactional
-    fun importZip(filename: String, bytes: ByteArray): ContentImportResult {
+    fun importZip(filename: String, bytes: ByteArray, sync: Boolean = true): ContentImportResult {
         if (!filename.lowercase().endsWith(".zip")) {
             throw BadRequestException("Ожидается .zip архив")
         }
@@ -509,8 +509,10 @@ class ContentService(
             artCount += 1
         }
 
-        // Синхронизация: в главах из архива удаляем разделы и статьи, которых в архиве нет.
-        for (chapterId in touchedChapters) {
+        // Синхронизация (если включена): в главах из архива удаляем разделы и статьи,
+        // которых в архиве нет. В режиме merge (sync = false) ничего не удаляем —
+        // удобно догружать отдельные статьи, не затрагивая правки, сделанные в интерфейсе.
+        for (chapterId in if (sync) touchedChapters else emptySet()) {
             for (section in sections.findByChapterIdOrderByPositionAscIdAsc(chapterId)) {
                 if (section.id in seenSections) {
                     for (article in articles.findBySectionIdOrderByPositionAscIdAsc(section.id)) {

services/academic-service/src/main/kotlin/ru/ysu/lms/academic/controller/ContentAdminController.kt

@@ -79,9 +79,12 @@ class ContentAdminController(private val contentService: ContentService) {
     }
 
     @PostMapping("/import")
-    fun import(@RequestParam("file") file: MultipartFile): ContentImportResult {
+    fun import(
+        @RequestParam("file") file: MultipartFile,
+        @RequestParam(value = "sync", defaultValue = "true") sync: Boolean,
+    ): ContentImportResult {
         if (file.isEmpty) throw BadRequestException("Выберите файл")
-        return contentService.importZip(file.originalFilename ?: "content.zip", file.bytes)
+        return contentService.importZip(file.originalFilename ?: "content.zip", file.bytes, sync)
     }
 
     @GetMapping("/articles")