feat(recurrence): repeating tasks as RRULE, in Danish and English

"vand planterne hver mandag" becomes FREQ=WEEKLY;BYDAY=MO. The stored
form is RFC 5545, not a private format, so the same field will drive the
CalDAV server and the ICS feed without a translation layer.

Ticking off a repeating task advances it instead of closing it, keeping
the same row — so its sub-tasks and comments stay attached. A weekly
review is one thing that recurs, not fifty-two things. The clock time
survives the move, and the completion is still written to the activity
log, because "what did I get done this week" has to include the chores
that repeat.

Recurrence is parsed before dates, and that ordering is load-bearing:
"hver mandag" contains a weekday, so a date parser reading it first
would claim Monday as a one-off and strand "hver" in the title. It is
also resolved before the time-only case, or "hver mandag kl 9" typed
after nine would land tomorrow morning instead of on Monday.

Co-Authored-By: Claude Opus 5 <[email protected]>

go.sum

@@ -27,6 +27,8 @@ github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qq
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.3.0 h1:TivCn/peBQ7UY8ooIcPgZFpTNSz0Q2U6UrFlUfqbe0Q=
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
+github.com/teambition/rrule-go v1.8.2 h1:lIjpjvWTj9fFUZCmuoVDrKVOtdiyzbzc93qTmRVe/J8=
+github.com/teambition/rrule-go v1.8.2/go.mod h1:Ieq5AbrKGciP1V//Wq8ktsTXwSwJHDD5mD/wLBGl3p4=
 golang.org/x/crypto v0.55.0 h1:+KWHjbgOaAQ66dh/YlkZKHlz9ZUlq61AFirAR9ntP8M=
 golang.org/x/crypto v0.55.0/go.mod h1:uq0V9dE/fzQuJtbnL+2EhWOE63vo164FY8xqEnV9xis=
 golang.org/x/mod v0.37.0 h1:vF1DjpVEshcIqoEaauuHebaLk1O1forxjxBaVn884JQ=