Schema Diff's generated migration SQL has no copy button

`src/components/SchemaDiff.tsx`'s "SQL Migration" view renders `generateMigrationSQL()`'s output in a bare `

` block with no way to copy it except a manual text selection, confirmed by reading the file. `src/components/copy-button.tsx` is a shared `CopyButton` already used for exactly this in `CodeGenerator.tsx`, `TestDataGenerator.tsx`, `DataImportModal.tsx` and `QueryEditor.tsx`.


**Done when:**

- The migration SQL view carries the shared `CopyButton`, matching how the other four call sites already use it.

- No new clipboard code is written for this.

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see [CONTRIBUTING.md](https://github.com/libredb/libredb-studio/blob/main/CONTRIBUTING.md).

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