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/

app/validators/Auth/register.ts

@@ -1,15 +0,0 @@
-import vine from '@vinejs/vine'
-
-export const registerValidator = vine.compile(
-  vine.object({
-    name: vine.string().minLength(3).maxLength(64),
-    email: vine
-      .string()
-      .email()
-      .unique(async (query, field) => {
-        const user = await query.from('users').where('email', field).first()
-        return !user
-      }),
-    password: vine.string().minLength(8).maxLength(32),
-  })
-)