Stats period selector: "This year"/"Last year"

Add new options on the stats period selector:

• This year (1st jan -> now)
• Last year (1st jan year-1 -> 31 dec year-1)
• This Month (1st of month -> now)
• Last Month (1st of prev month -> end of prev month)

Requested:

• Survey 2026
• https://www.reddit.com/r/DiapStash/comments/1r8y12d/add_last_year_and_this_year_to_date_options_for/