- Replace global `is_empty_for_first_import()` with independent checks:
- `is_mcp_table_empty()` for MCP server imports
- `is_prompts_table_empty()` for prompt imports
- Skills and providers already have built-in idempotency checks
- Fix misleading logs in provider import:
- Change `import_default_config` return type from `Result<()>` to `Result<bool>`
- Return `true` when actually imported, `false` when skipped
- Only log success message when import actually occurred
- Add idempotency protection to `import_from_file_on_first_launch`
This allows each data type to be independently recovered if deleted,
rather than requiring all tables to be empty for any import to trigger.