refactor: 重构后端目录结构和命名规范

- 将后端目录名从大写改为小写 (Controller->controller 等)
- 将命名空间从 App\ 改为 app\ (符合 webman 官方规范)
- 移除不必要的 bootstrap 目录
- 完善出差审批业务功能：
  - 添加删除草稿功能
  - 添加审批操作功能（同意/拒绝）
  - 完善审批详情页，显示审批人和审批记录
- 前端优化：
  - 调整列表页行高
  - 隐藏底部导航栏
  - 仪表盘只显示3个快捷入口

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/model/BusinessTrip.php

@@ -0,0 +1,101 @@
+<?php
+
+namespace app\model;
+
+use Illuminate\Database\Eloquent\Model;
+
+class BusinessTrip extends Model
+{
+    protected $table = 'business_trip';
+
+    protected $fillable = [
+        'user_id',
+        'reason',
+        'start_date',
+        'end_date',
+        'destination',
+        'estimated_cost',
+        'status',
+        'approval_order_id',
+    ];
+
+    protected $casts = [
+        'user_id' => 'integer',
+        'estimated_cost' => 'decimal:2',
+        'created_at' => 'datetime',
+        'updated_at' => 'datetime',
+    ];
+
+    /**
+     * 状态常量
+     */
+    const STATUS_DRAFT = 'draft';         // 草稿
+    const STATUS_PENDING = 'pending';     // 审批中
+    const STATUS_APPROVED = 'approved';   // 已通过
+    const STATUS_REJECTED = 'rejected';   // 已拒绝
+    const STATUS_CANCELLED = 'cancelled'; // 已撤销
+
+    /**
+     * 关联用户
+     */
+    public function user()
+    {
+        return $this->belongsTo(User::class, 'user_id');
+    }
+
+    /**
+     * 关联行程明细
+     */
+    public function itineraries()
+    {
+        return $this->hasMany(BusinessTripItinerary::class, 'business_trip_id');
+    }
+
+    /**
+     * 关联审批单
+     */
+    public function approvalOrder()
+    {
+        return $this->belongsTo(ApprovalOrder::class, 'approval_order_id');
+    }
+
+    /**
+     * 计算总天数
+     */
+    public function getDurationDays(): int
+    {
+        if (!$this->start_date || !$this->end_date) {
+            return 0;
+        }
+        $start = \DateTime::createFromFormat('Y-m-d', $this->start_date);
+        $end = \DateTime::createFromFormat('Y-m-d', $this->end_date);
+        if (!$start || !$end) {
+            return 0;
+        }
+        return $end->diff($start)->days + 1;
+    }
+
+    /**
+     * 判断是否可以编辑
+     */
+    public function canEdit(): bool
+    {
+        return $this->status === self::STATUS_DRAFT;
+    }
+
+    /**
+     * 判断是否可以提交
+     */
+    public function canSubmit(): bool
+    {
+        return $this->status === self::STATUS_DRAFT;
+    }
+
+    /**
+     * 判断是否可以取消
+     */
+    public function canCancel(): bool
+    {
+        return $this->status === self::STATUS_PENDING;
+    }
+}