Add LogZero to Productivity

[and1hof]

Adds LogZero to the Productivity section, alongside other habit trackers (Streaks, Timpler).

LogZero is an all-in-one private personal life and habit tracker for iOS (mood, medication, food, exercise, meditation, weight, to-dos, plus a local correlation engine and streaks dashboard). It is 100% on-device with no account, no ads, and no analytics/telemetry, and is App Store labelled "Data Not Collected".

• Website: https://logzero.app
• App Store: https://apps.apple.com/app/id6773557137

Entry added:

- [LogZero](https://logzero.app) - On-device habit and mood tracker with no account, ads, or trackers.

Checklist against the contributing guidelines:

• Searched existing entries — not a duplicate.
• Format - [Name](link) - Description. followed.
• Placed in the closest existing category (Productivity).
• Description starts with a capital, ends with a full stop, and does not start with "A"/"An".
• Spelling and grammar checked.