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]>

app/Models/Settings/ProjectSetting.php

@@ -16,4 +16,9 @@ class ProjectSetting extends Model
     {
         return $query->where('is_active', true);
     }
+
+    public function locations(): \Illuminate\Database\Eloquent\Relations\HasMany
+    {
+        return $this->hasMany(\App\Models\Settings\Location::class, 'project_id');
+    }
 }