新增單位管理以及一些功能修正

database/migrations/2026_01_08_103000_create_units_table.php

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+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('units', function (Blueprint $table) {
+            $table->id();
+            $table->string('name')->unique()->comment('單位名稱');
+            $table->string('code')->nullable()->comment('單位代碼 (如: kg)');
+            $table->timestamps();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('units');
+    }
+};

database/migrations/2026_01_08_103500_update_products_table_units.php

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+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('products', function (Blueprint $table) {
+            // Drop old string columns
+            $table->dropColumn(['base_unit', 'large_unit', 'purchase_unit']);
+
+            // Add new foreign key columns
+            $table->foreignId('base_unit_id')->nullable()->after('specification')->constrained('units')->nullOnDelete()->comment('基本庫存單位ID');
+            $table->foreignId('large_unit_id')->nullable()->after('base_unit_id')->constrained('units')->nullOnDelete()->comment('大單位ID');
+            $table->foreignId('purchase_unit_id')->nullable()->after('conversion_rate')->constrained('units')->nullOnDelete()->comment('採購單位ID');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('products', function (Blueprint $table) {
+            // Remove foreign keys
+            $table->dropForeign(['base_unit_id']);
+            $table->dropForeign(['large_unit_id']);
+            $table->dropForeign(['purchase_unit_id']);
+            $table->dropColumn(['base_unit_id', 'large_unit_id', 'purchase_unit_id']);
+
+            // Add back string columns (nullable since data is lost)
+            $table->string('base_unit')->nullable()->comment('基本庫存單位 (e.g. g, ml)');
+            $table->string('large_unit')->nullable()->comment('大單位 (e.g. 桶, 箱)');
+            $table->string('purchase_unit')->nullable()->comment('採購單位');
+        });
+    }
+};