Improve shared water sync across platforms

[TienNguyen3711]

Summary

• improve shared hydration sync between web and mobile by making water intake writes idempotent per user and date
• normalize daily water intake updates so repeated writes update the same record instead of creating diverging entries
• make cross-platform hydration reads more reliable by supporting a single source of truth for the current day

What files changed

• routes/waterIntake.js

Testing

• reviewed GET /api/water-intake and POST /api/water-intake flow against authenticated user scoping
• verified route syntax with:
  • node -c routes/waterIntake.js
• manually confirmed the route now:
  • resolves the authenticated user for normal requests
  • normalizes the request date
  • updates the existing daily row when one already exists
  • inserts only when no row exists yet
• not run end-to-end across clients in this final pass