feat: v2 Vue3 frontend + multiple optimizations

- New Vue 3 + Vite frontend at /v2/ (OLED dark theme, Fira Sans/Code)
- Date selector: support day/week/month range (backend unchanged)
- SSE auto-reconnect (up to 3 retries)
- Visibility polling pause (dashboard pauses when tab hidden)
- Friendly Chinese HTTP error messages
- Cancel task with confirmation in Dashboard
- Split AiWorkflowService (1700->845 lines):
  - AiApiService: AI API calls + streaming
  - ExcelExportService: POI Excel generation
- Dockerfile: 3-stage build (Node frontend -> Maven -> JRE)
- WebApplication.java: System.out -> Logger
- .gitignore: v2 build output, backup dirs

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -1,3 +1,12 @@
+# frontend-vue (Vue 3 + Vite)
+frontend-vue/node_modules/
+
+# frontend build output (auto-generated by Docker build)
+src/main/resources/static/v2/
+
+# Backup directory
+outputs.nobody-backup-*/
+
 # Maven
 target/
 pom.xml.tag