Sudoku
Standard rules — each row, column and 3×3 box gets every digit 1–9 once. Tap a cell then a number; conflicts turn red right away. Toggle Notes to pencil in candidates instead of placing a digit. Same puzzle for everyone; the streak counts days played, not days won.
What the difficulty setting does
Five levels, from 46 given numbers down to 32. They are not five different puzzles: the day's grid is generated from the date alone, and a difficulty simply decides how much of that one solution you are shown. An easier level's clues are a superset of a harder one's, so switching down mid-week is never a different target — it is the same puzzle with more of it filled in.
Which is also why the streak ignores the setting entirely. It counts days played, at whatever level you played them.
Signed out: this one needs an account to play. Every round is dealt and settled on the server against a user, so there is no guest version to offer — the overlay dismisses so you can read the page and see what you would be signing up for.