UI: 预览环境显示构建时间 + 售价默认299

- 预览栏显示部署时间（如 04/11 10:05）
- 售价默认299
- vite构建时注入__BUILD_TIME__

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/Projects.vue

@@ -304,7 +304,7 @@ function selectDemoProject() {
     packaging_cost: p.packaging_cost || 0,
     labor_cost: p.labor_cost || 0,
     other_cost: p.other_cost || 0,
-    selling_price: p.selling_price || 0,
+    selling_price: p.selling_price || 299,
     quantity: p.quantity || 1,
     notes: p.notes || '',
   }
@@ -330,7 +330,7 @@ async function createProject() {
         packaging_cost: 0,
         labor_cost: 0,
         other_cost: 0,
-        selling_price: 0,
+        selling_price: 299,
         quantity: 1,
         notes: '',
       }),
@@ -357,7 +357,7 @@ function selectProject(p) {
     packaging_cost: p.packaging_cost || 0,
     labor_cost: p.labor_cost || 0,
     other_cost: p.other_cost || 0,
-    selling_price: p.selling_price || 0,
+    selling_price: p.selling_price || 299,
     quantity: p.quantity || 1,
     notes: p.notes || '',
   }