Add “Raw RRULE” field and clarify COUNT field [#3540328]

Problem/Motivation

Users assume rrule_field accepts raw RRULE strings, but the code only treats numeric input as COUNT. This silent mismatch breaks recurrence rules.

Steps to reproduce

Create a feed and map rrule_field to a field with the value
RRULE:FREQ=WEEKLY;INTERVAL=1;BYDAY=FR;UNTIL=20250831T065959Z
Leave rrule unmapped.
Preview View to see no RRULE

Proposed resolution

Keep existing rrule_field (numeric only, COUNT).
Label: “Number of occurrences (COUNT)” or something to indicate it relates to the rrule frequency field (maybe #states).
Add new mapping key rrule_raw_field for full RRULE strings.
Label: “Raw RRULE string”.
In DateICal::feed() use rrule_raw_field when present; otherwise fall back to current logic.
Document both fields in UI help.

User interface changes

New select option “Raw RRULE string”.
Relabel existing COUNT widget.

Issue fork date_ical-3540328

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3540328-add-raw-rrule changes, plain diff MR !7
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