Fix Default Line Width

[neil-lobo]

fixes: #42 fixes: #46 fixes: #48

related PR: #43

This comment is incorrect: https://github.com/zhanglun/lettura/pull/43#issuecomment-2046965975

Although you set a default CSS value in the CSS itself, it is also set after loading the user config here: https://github.com/zhanglun/lettura/blob/023d6fc7c90dd1c95dd43d817e92d09073a3c0e4/src/App.tsx#L52-L59

Since the default is set to 1, that is what is used when creating the initial lettura.toml file. https://github.com/zhanglun/lettura/blob/023d6fc7c90dd1c95dd43d817e92d09073a3c0e4/src-tauri/src/core/config.rs#L29-L38

The new default value is taken from this piece of code: https://github.com/zhanglun/lettura/blob/023d6fc7c90dd1c95dd43d817e92d09073a3c0e4/src/layout/Setting/CustomizeStyle/index.tsx#L20-L26

Without this change, every new user will face the issues linked in this PR.