fix(ics): harden the import against foreign files (#225)

Review pass over the Fossify import work:

- sanitizeRrule whitelists the parts EventRecurrence actually parses, and
  reports whether it dropped anything — normalising case or a stray ";" no
  longer warns the user about a faulty repeat rule.
- CATEGORIES only stands in for a calendar name when it is single-valued and
  every event agrees; elsewhere it is a tag list and must not pick the target.
- An all-day EXDATE keeps the day it spells out instead of the day its instant
  lands on in UTC.
- An imported all-day reminder is sampled where it will fire, not at a
  recurrence anchor that Fossify dates to 1970.
- Imported colours match against every published key, not the picker's
  curated subset.

CHANGELOG.md

@@ -26,7 +26,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **All-day reminders survive a backup and an import.** They fire at the hour you
   chose, not at midnight UTC — the same rule the app already applied to all-day
   events you create yourself — and an export no longer leaves them out of the
-  file entirely ([#225]).
+  file entirely. An imported yearly birthday fires at the right hour too, even
+  though the file dates it back to the year of birth ([#225]).
 - **Birthdays and anniversaries imported from Fossify Calendar now show up.**
   Fossify writes the ones it mirrors from your contacts as events that start and
   end on the same day, which the calendar read as lasting no time at all: they