Add Lifestack to Productivity showcase

[wakkihaya]

Summary

• Adds Lifestack (https://lifestack.ai) to the Productivity category

Lifestack is the smart daily planner built around your energy. It reads your sleep, recovery, and focus, then drafts a schedule that puts the right work at the right time. The desktop app is available thanks to electron

Test plan

• Ran mocha test/human-data.js locally against this entry (19 passing, 0 failing)
• Ran oxfmt --check on the new yml file
• Verified rendering locally against a patched electron/website dev build — card displays correctly under the Productivity filter with logo, name, description, and working website link