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>

docs/README_Web.md

@@ -93,6 +93,7 @@ http://localhost:18088
 
 - `POST /api/svn/test-connection`
 - `POST /api/svn/fetch`
+- `POST /api/svn/version-range`：按 `rangeType=date|week|month` 查询版本范围；旧的 `year/month` 月份请求仍兼容
 - `GET /api/svn/presets`
 - `POST /api/ai/analyze`
 - `GET /api/tasks`