gui: drag-and-drop sidebar, tree expand, localization fixes

TreeNode.svelte:
- Native HTML5 drag-and-drop with move effect
- Lazy tree expand/collapse (arrow for container types only)
- Drop validation: no self-drop, container-only, descendant check
- 'case' icon kind added

App.svelte:
- toggleExpand loads children via ListChildren into tree
- handleNodeDrop calls MoveNode(draggedId, targetId), refreshes tree
- Root workspace area is a drop target (handleDropRoot)
- Overview section shows nodeKindLabel instead of raw type enum
- Context menu shows Create only for container types
- Create modal title uses 'Создать элемент'
- submitCreateNode expands parent after child creation

TemplateIcon.svelte: added 'case' icon (folder-like with dividers)
i18n: added nav.createNode key (ru+en)

frontend/src/lib/TemplateIcon.svelte

@@ -32,6 +32,11 @@
   {:else if kind === 'file'}
     <path d="M13 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V9z"/>
     <polyline points="13 2 13 9 20 9"/>
+  {:else if kind === 'case'}
+    <rect x="3" y="5" width="18" height="14" rx="2"/>
+    <polyline points="3 10 21 10"/>
+    <line x1="8" y1="5" x2="8" y2="19"/>
+    <line x1="16" y1="5" x2="16" y2="19"/>
   {:else}
     <circle cx="12" cy="12" r="10"/>
     <line x1="12" y1="16" x2="12" y2="12"/>

frontend/src/lib/i18n/locales/en.js

@@ -15,6 +15,7 @@ export default {
   'nav.noNodes': 'No nodes',
   'nav.openFolder': 'Open folder',
   'nav.createInside': 'Create inside',
+  'nav.createNode': 'Create element',
   'nav.selectPrompt': 'Select a section or case',
   'nav.brand': 'Verstak',
 

frontend/src/lib/i18n/locales/ru.js

@@ -20,6 +20,7 @@ export default {
   'nav.noNodes': 'Нет узлов',
   'nav.openFolder': 'Открыть папку',
   'nav.createInside': 'Создать внутри',
+  'nav.createNode': 'Создать элемент',
 
   'tab.overview': 'Обзор',
   'tab.notes': 'Заметки',