ByteDance Ltd. plans to buy back employee stock at a valuation of about $312 billion, a significant markup from previous levels that reflects a brightening outlook for Chinese tech shares.TikTok’s owner is offering US staff about $189.90 a share, according to a person familiar with the deal. That’s up from about $181 in a similar proposal from about six months ago, the person said, asking not to be identified discussing an internal decision....