fix(import): make .ics import survive events the provider rejects (#225)

importEvents inserted every event in one unguarded loop, so a single row
the provider refused — it throws on a malformed RRULE straight out of
insert — aborted the batch and left the user with "couldn't read this
file" and nothing imported. Each event is now isolated and rejects are
counted into IcsImportSummary.failed and shown. sanitizeRrule drops empty
and malformed rule parts before the write.

Alongside that, several things a foreign file carries that we dropped:
VTODO components (silently lost, now imported as events), EXDATE, the
CATEGORIES calendar name, and colours — X-FOSSIFY-EVENT-COLOR / COLOR /
the category colour plus the X-SMT-* legacy spellings, snapped in Oklab
to the nearest key a palette account publishes since those reject a raw
EVENT_COLOR.

Two correctness fixes on our side: an all-day event may no longer end at
or before it starts (the provider expands a zero-length series into no
instances, so it just disappears), and imported all-day reminders now go
through the same encoding as hand-created ones instead of firing at UTC
midnight. A VALARM trigger pointing after the start is read as a time of
day rather than clamped to zero.

Note for later: their all-day DTEND is RFC-correct — endTS anchors at
noon of the last day and the exporter's +12h rounds it to the following
midnight. Do not "fix" it by sniffing PRODID; the holiday files bundled
in Fossify are conformant and name Fossify in theirs. One is kept as a
fixture.

Refs #225

app/src/test/resources/ics/fossify-holidays-at.ics

@@ -0,0 +1,25 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+CALSCALE:GREGORIAN
+PRODID:Fossify Calendar Holiday Generator
+METHOD:PUBLISH
+X-PUBLISHED-TTL:PT1H
+BEGIN:VEVENT
+UID:fossify_c10f9ad245dc2e57fb3652abee6ed8a06744f0bd
+SUMMARY:Neujahr
+DTSTAMP:20260526T132315Z
+DTSTART;VALUE=DATE:19800101
+DTEND;VALUE=DATE:19800102
+STATUS:CONFIRMED
+RRULE:FREQ=YEARLY
+END:VEVENT
+BEGIN:VEVENT
+UID:fossify_8a60ccf65b65265f768dd33a0b7aefc7620e3ee2
+SUMMARY:Heilige Drei Könige
+DTSTAMP:20260526T132315Z
+DTSTART;VALUE=DATE:19800106
+DTEND;VALUE=DATE:19800107
+STATUS:CONFIRMED
+RRULE:FREQ=YEARLY
+END:VEVENT
+END:VCALENDAR