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>

frontend-vue/vite.config.js

@@ -0,0 +1,26 @@
+import { defineConfig } from 'vite'
+import vue from '@vitejs/plugin-vue'
+import { resolve } from 'path'
+
+export default defineConfig({
+  plugins: [vue()],
+  base: '/v2/',
+  resolve: {
+    alias: {
+      '@': resolve(__dirname, 'src'),
+    },
+  },
+  server: {
+    port: 5173,
+    proxy: {
+      '/api': {
+        target: 'http://localhost:8080',
+        changeOrigin: true,
+      },
+    },
+  },
+  build: {
+    outDir: resolve(__dirname, '..', 'src', 'main', 'resources', 'static', 'v2'),
+    emptyOutDir: true,
+  },
+})