feat: Add Kits + FurnaceSplitter plugin config modules

DB migrations 016 (kits_configs) and 019 (furnacesplitter_configs) applied.
Backend: NestJS modules with CRUD, apply-to-server, import-from-server.
Frontend: Pinia stores, Vue views with config editor, router + nav wiring.
Kits view: 3-tab editor (list/editor/settings), kit items with shortname/amount/skinId/container.
FurnaceSplitter view: per-furnace toggles, split count, fuel multiplier settings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend-nest/src/modules/furnacesplitter/dto/create-furnacesplitter-config.dto.ts

@@ -0,0 +1,19 @@
+import { IsString, IsOptional, IsObject, MaxLength } from 'class-validator';
+import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';
+
+export class CreateFurnaceSplitterConfigDto {
+  @ApiProperty({ example: 'Default FurnaceSplitter' })
+  @IsString()
+  @MaxLength(100)
+  config_name: string;
+
+  @ApiPropertyOptional({ example: 'Standard furnace splitter settings' })
+  @IsString()
+  @IsOptional()
+  description?: string;
+
+  @ApiPropertyOptional()
+  @IsObject()
+  @IsOptional()
+  config_data?: Record<string, any>;
+}