feat: add Projects settings with sub-locations and cascading dropdowns in purchase request modal

- Migration: add project_id FK to settings_locations
- Models: ProjectSetting hasMany Location, Location belongsTo ProjectSetting
- Settings: /settings/projects page — manage projects and their sub-locations (two-panel UI)
- Sidebar: Projects nav item under Settings group
- Routes: 7 new settings/projects routes (Admin only)
- Modal: project_name and location fields now cascading dropdowns populated from settings_projects/settings_locations

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

database/migrations/2026_05_25_000000_add_project_id_to_settings_locations_table.php

@@ -0,0 +1,30 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('settings_locations', function (Blueprint $table) {
+            $table->foreignId('project_id')
+                  ->nullable()
+                  ->after('name')
+                  ->constrained('settings_projects')
+                  ->nullOnDelete();
+
+            $table->index('project_id');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('settings_locations', function (Blueprint $table) {
+            $table->dropIndex(['project_id']);
+            $table->dropForeign(['project_id']);
+            $table->dropColumn('project_id');
+        });
+    }
+};