Fix 500 on REPORT for recurring events with Windows timezone IDs

DAVx5 reported a 500 Internal Server Error on a time-range REPORT
against a calendar containing a recurring event whose DTSTART/DTEND used
a Windows-style TZID (e.g. "W. Europe Standard Time", as commonly written
by Outlook/Exchange and some Thunderbird/Lightning setups) instead of an
IANA zone name.

go-ical resolves TZID via a plain time.LoadLocation call, which only
understands IANA names. A simple decode of such an event succeeds (RRULE
dates aren't parsed eagerly), but expanding its recurrence - which
go-webdav's caldav.Filter does for every time-range REPORT - calls
Component.RecurrenceSet, which does call time.LoadLocation(tzid) and
fails with "ical: error parsing start time: unknown time zone ...".

Add internal/icalfix, a small shared helper that rewrites recognized
Windows timezone identifiers (TZID parameters and VTIMEZONE TZID: lines)
to their IANA equivalent in raw ICS bytes before decoding. Wire it into
every ical.NewDecoder call site: internal/caldav/backend.go's
decodeObject (fixes the reported bug), internal/web/calendar.go's event
rendering/ICS import, and internal/icssub's remote feed fetching, so a
subscribed feed with the same issue doesn't hit it either.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/icalfix/icalfix_test.go

@@ -0,0 +1,71 @@
+package icalfix
+
+import (
+	"bytes"
+	"testing"
+
+	ical "github.com/emersion/go-ical"
+)
+
+// TestNormalizeTimeZonesFixesRecurringEvent reproduces the bug reported by
+// a DAVx5 client: a recurring VEVENT with a Windows-style TZID
+// ("W. Europe Standard Time") decodes fine, but expanding its recurrence
+// via RecurrenceSet (as go-webdav's caldav.Filter does for a time-range
+// REPORT) fails with "unknown time zone" because go-ical resolves TZID
+// via a plain time.LoadLocation call. Normalizing the TZID to its IANA
+// equivalent before decoding must make that expansion succeed.
+func TestNormalizeTimeZonesFixesRecurringEvent(t *testing.T) {
+	const raw = "BEGIN:VCALENDAR\r\n" +
+		"VERSION:2.0\r\n" +
+		"PRODID:-//Test//Test//EN\r\n" +
+		"BEGIN:VTIMEZONE\r\n" +
+		"TZID:W. Europe Standard Time\r\n" +
+		"END:VTIMEZONE\r\n" +
+		"BEGIN:VEVENT\r\n" +
+		"UID:recurring@example.com\r\n" +
+		"DTSTAMP:20240101T000000Z\r\n" +
+		"DTSTART;TZID=W. Europe Standard Time:20260522T220000\r\n" +
+		"DTEND;TZID=W. Europe Standard Time:20260522T230000\r\n" +
+		"RRULE:FREQ=WEEKLY;COUNT=5\r\n" +
+		"SUMMARY:Weekly sync\r\n" +
+		"END:VEVENT\r\n" +
+		"END:VCALENDAR\r\n"
+
+	normalized := NormalizeTimeZones([]byte(raw))
+
+	cal, err := ical.NewDecoder(bytes.NewReader(normalized)).Decode()
+	if err != nil {
+		t.Fatalf("decoding normalized calendar: %v", err)
+	}
+	events := cal.Events()
+	if len(events) != 1 {
+		t.Fatalf("expected 1 event, got %d", len(events))
+	}
+
+	if _, err := events[0].RecurrenceSet(nil); err != nil {
+		t.Fatalf("expanding recurrence after normalization: %v", err)
+	}
+
+	// The un-normalized original must reproduce the reported failure, so
+	// this test actually demonstrates the fix rather than trivially
+	// passing regardless.
+	origCal, err := ical.NewDecoder(bytes.NewReader([]byte(raw))).Decode()
+	if err != nil {
+		t.Fatalf("decoding original calendar: %v", err)
+	}
+	if _, err := origCal.Events()[0].RecurrenceSet(nil); err == nil {
+		t.Fatalf("expected RecurrenceSet to fail on un-normalized Windows TZID, it didn't")
+	}
+}
+
+func TestNormalizeTimeZonesLeavesUnknownAndIANAZonesAlone(t *testing.T) {
+	const raw = "BEGIN:VCALENDAR\r\n" +
+		"DTSTART;TZID=Europe/Berlin:20260522T220000\r\n" +
+		"DTEND;TZID=Some/Unknown-Zone:20260522T230000\r\n" +
+		"END:VCALENDAR\r\n"
+
+	got := string(NormalizeTimeZones([]byte(raw)))
+	if got != raw {
+		t.Fatalf("expected data to be unchanged, got:\n%s", got)
+	}
+}