refactor: uniform validation, fix create form UX, dedupe filter dropdowns

- Remove dead registerValidator (Auth/register.ts), unused since signupValidator
  is the one actually wired to NewAccountController
- computers/create.vue: switch to useForm() + @submit.prevent so the form
  submits on Enter and shows a processing state, matching components/create.vue
- components/edit.vue: drop the 'origin' field, which was never provided by
  the backend (edit() never shared it) and therefore always undefined
- Extract FilterDropdown.vue and reuse it in ComponentsViewer/ComputersViewer,
  removing ~80 lines of duplicated dropdown markup/logic and fixing duplicate
  HTML ids (id="used" appeared twice in ComputersViewer)
- Move MIGRATION_*.md artifacts into docs/migration-history/

.adonisjs/server/pages.d.ts

@@ -37,5 +37,6 @@ declare module '@adonisjs/inertia/types' {
     'widgets/SimpleGallery': ExtractProps<(typeof import('../../inertia/pages/widgets/SimpleGallery.vue'))['default']>
     'widgets/TableComponent': ExtractProps<(typeof import('../../inertia/pages/widgets/TableComponent.vue'))['default']>
     'widgets/ValidationError': ExtractProps<(typeof import('../../inertia/pages/widgets/ValidationError.vue'))['default']>
+    'widgets/FilterDropdown': ExtractProps<(typeof import('../../inertia/pages/widgets/FilterDropdown.vue'))['default']>
   }
 }