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.

internal/core/storage/migrations_004.sql.go

@@ -0,0 +1,9 @@
+package storage
+
+// migration004 — add section column to nodes for sidebar grouping.
+// Valid sections: clients, projects, recipes, documents, archive, inbox.
+// NULL = unassigned (inbox).
+const migration004 = `
+ALTER TABLE nodes ADD COLUMN section TEXT NULL;
+CREATE INDEX IF NOT EXISTS idx_nodes_section ON nodes(section);
+`

internal/core/storage/storage.go

@@ -60,7 +60,8 @@ var migrationFiles = map[int]string{
 	1: migration001,
 	2: migration002,
 	3: migration003,
-	// 4: migration004, etc.
+	4: migration004,
+	// 5: migration005, etc.
 }
 
 func (db *DB) runInitialSchema() error {