Private local-first health stat logging (spreadsheet → lightweight PWA)

Many of the estimated 400 million active self-tracking users struggle with logging everyday health stats because mainstream apps are cloud-first, heavy, require accounts or subscriptions, and make it hard to export data into familiar formats like spreadsheets; this pain is especially acute for people managing chronic conditions (hypertension, diabetes), privacy-conscious users, and clinicians or caregivers who need simple CSV exports for review. The result is fragmented, locked-in data and low trust in app ecosystems despite frequent home monitoring. A practical product would be a spreadsheet-centric, local-first Progressive Web App that stores entries in-browser (IndexedDB) with optional end-to-end encrypted sync, supports CSV/ODS import-export, Web Bluetooth/USB for pairing common home devices, offline-first charts, and lightweight templated dashboards for clinicians. Pricing can follow a freemium model with a $30/year ARPU target for premium features (encrypted backup, automated exports, clinician sharing, device integrations), aligning with the $12.0B global consumer digital health apps market implied by 400M users × $30 ARPU. The market timing is favorable because home monitoring adoption is growing and privacy-first consumer preferences are rising, while browser APIs and PWAs have matured enough to enable cross-platform, installable experiences without app-store friction. Differentiation comes from being genuinely local-first (no account required), spreadsheet-native for low-friction interoperability, and lightweight as a PWA, but real challenges include reliable device integrations, demonstrating clinical credibility, and converting users in a space with free incumbents. A sensible early plan is to validate with 10,000 paying power users (~$300k/year) and a few clinical partners to refine device support and workflow before scaling.