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/favicon.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <rect x="2" y="2" width="28" height="28" rx="6" fill="#6366f1"/>
+  <path d="M9 8v13h5l-1.5 3H6V8z" fill="#eef2ff"/>
+  <path d="M23 8v13h-5l1.5 3h5V8z" fill="#c7d2fe"/>
+  <path d="M11 9c2-1.5 4-1.5 5 0 1-1.5 3-1.5 5 0v10c-2-1.5-4-1.5-5 0-1-1.5-3-1.5-5 0z" fill="#ffffff"/>
+  <path d="M16 9v10" stroke="#6366f1" stroke-width="1"/>
+</svg>