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>

maven-settings.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<settings xmlns="http://maven.apache.org/SETTINGS/1.0.0"
+          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+          xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0
+          http://maven.apache.org/xsd/settings-1.0.0.xsd">
+    <!--
+        阿里云 Maven 镜像配置
+        将 Maven Central 所有请求代理到阿里云国内镜像，大幅加速依赖下载
+    -->
+    <mirrors>
+        <mirror>
+            <id>aliyun-maven</id>
+            <name>阿里云 Maven 镜像 (Aliyun Maven Mirror)</name>
+            <url>https://maven.aliyun.com/repository/public</url>
+            <mirrorOf>central</mirrorOf>
+        </mirror>
+    </mirrors>
+</settings>