feat: node section assignment for sidebar filtering + search fix

Backend:
- Migration 004: add 'section' column to nodes table
  (NULL=inbox, values: clients/projects/recipes/documents/archive)
- Create(parentID, type, title, section) — section stored on root nodes
- ListRoots(includeDeleted, section) — filters by section
  (section='inbox' returns nodes with NULL section)
- GET /api/nodes?section=X filters root nodes by section
- POST /api/nodes accepts 'section' field in body

Frontend:
- Sidebar separates 'НАВИГАЦИЯ' (virtual sections) from 'ДЕЛА' (real nodes)
- Each section loads only its own nodes: GET /api/nodes?section=clients etc.
- Creating from a section sets the section automatically
- Inbox shows only nodes with no section
- selectBySearch(id) closes result dropdown after selection
- All types shown in Russian (Дело, Заметка, Папка, etc.)

Acceptance: go build pass, go test pass (all packages),
  manual: Pro projects section shows only project-nodes,
  clients only client-nodes, inbox only unsectioned nodes.

docs/PLAN.md

@@ -16,7 +16,7 @@
 | 3 | Nodes Repository + CRUD + CLI Node | ✅ выполнен |
 | 4 | Vault Files: Trash + File Service + CLI File | ✅ выполнен |
 | 5 | Markdown Notes: Create/Read/Save + CLI Note | ✅ выполнен |
-| 6 | Wails GUI MVP: Sidebar + Main Panel | ✅ выполнен |
+| 6 | Wails GUI MVP: Sidebar + Main Panel | ✅ выполнен (Go HTTP SPA) |
 | 7 | Actions: Run URL/File/Command + GUI Tab | ⬜ не начат |
 | 8 | Worklog: Entries + Report + GUI Tab | ⬜ не начат |
 | 9 | FTS5 Search: Rebuild Index + GUI Search Bar | ⬜ не начат |