UI rework: modal dialogs, inline lab editing, manual creation, content management

- Dashboard: real stats (groups, students, labs, average grade), labs/groups
  summary lists, no placeholder tile
- Lab page: view + edit combined — metadata form in modal, condition rendered
  as markdown, per-level variant tables with add/edit/delete dialogs,
  level rename/delete, student level/variant selectors with group filter
- Labs list: create-lab dialog with manual level entry (code; title lines)
- Groups: create-group dialog, add-student dialog (auto password, toast card)
- Content: public /learn reader is the single place; admin buttons (new
  article with chapter/section create-or-select, rename/delete chapters and
  sections, move articles between sections, import zip) appear for admins;
  removed separate 'Контент' admin section
- Buttons use btn classes instead of nav-link
- Favicon: generated SVG book icon
- Security: CSRF synchronizer token interceptor (replaced spring-security in
  web which broke logback under nested jars), session regenerated on login,
  hardened session cookie, gateway actuator limited to health,info
- Jackson 3: nullable-safe request DTOs for labs (FAIL_ON_NULL_FOR_PRIMITIVES)
- academic: POST /labs (manual create with levels), level add/rename/delete,
  content chapters/sections CRUD, article create/move/delete endpoints

web/src/main/resources/static/css/app.css

@@ -167,6 +167,27 @@ tr:last-child td { border-bottom: none; }
 .filter-select { width: auto; }
 .hidden { display: none !important; }
 
+/* Toast-уведомление */
+.toast { position: fixed; top: 1rem; right: 1rem; z-index: 50; width: 20rem; }
+
+/* Modals */
+dialog.modal {
+    border: none; border-radius: 0.75rem; padding: 0;
+    background: var(--c-surface); color: var(--c-text);
+    width: min(34rem, calc(100vw - 2rem));
+    box-shadow: 0 20px 60px rgba(0,0,0,0.35);
+}
+dialog.modal::backdrop { background: rgba(0,0,0,0.55); }
+.modal-header { display: flex; align-items: center; justify-content: space-between; padding: 1rem 1.5rem; border-bottom: 1px solid var(--c-border); }
+.modal-title { margin: 0; font-size: 1.0625rem; font-weight: 600; }
+.modal-close { border: none; background: none; cursor: pointer; font-size: 1.25rem; color: var(--c-text-muted); padding: 0 0.25rem; }
+.modal-close:hover { color: var(--c-text); }
+.modal-body { padding: 1.5rem; max-height: 70vh; overflow-y: auto; }
+
+/* Двухуровневые формы уровней лабы */
+.level-row { display: flex; gap: 0.5rem; align-items: center; margin-bottom: 0.5rem; }
+.level-row .input { flex: 1; }
+
 /* Markdown body */
 .md-body { font-size: 0.9375rem; }
 .md-body h1, .md-body h2, .md-body h3 { margin: 1rem 0 0.5rem; line-height: 1.3; }