calendar: read a wall clock as a wall clock, and unfold iCal (V-581)

FactSpan built both instants by adding a duration to local midnight, so on
the two DST changeover days every span was an hour off. A day is 23 or 25
hours wide there, and the busy gate then read a 14:00 meeting as 13:00 or
15:00. Both readings are time.Date now, and the midnight crossing is AddDate
rather than a 24-hour add.

The iCal parse did not unfold content lines. A server folds a property at 75
octets and a Russian summary is two bytes a letter, so the tail of an
ordinary weekly standup was read as an unknown property and dropped, and the
event was filed under a truncated name. RFC 5545 TEXT escapes are also
reversed now, which RenderICal has always written and the parse never undid.

Two regression tests: a folded and escaped summary, and a span across the
start of DST in Europe/Berlin.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/calendar/ical_render_test.go

@@ -61,6 +61,44 @@ func TestRenderICalEscapesInjection(t *testing.T) {
 	}
 }
 
+// A folded SUMMARY is one property, not a property plus a dropped tail. Servers
+// fold at 75 octets and a Russian summary is two bytes a letter.
+func TestParseICalUnfoldsAndUnescapes(t *testing.T) {
+	body := []byte("BEGIN:VEVENT\r\n" +
+		"UID:u1\r\n" +
+		"DTSTART:20260703T130000Z\r\n" +
+		"DTEND:20260703T140000Z\r\n" +
+		"SUMMARY:Еженедельная планёрка\\, потом\r\n  созвон\r\n" +
+		"END:VEVENT\r\n")
+	from := time.Date(2026, 7, 3, 0, 0, 0, 0, time.UTC)
+	events := ParseICal(body, from, from.AddDate(0, 0, 1))
+	if len(events) != 1 {
+		t.Fatalf("got %d events, want 1", len(events))
+	}
+	if want := "Еженедельная планёрка, потом созвон"; events[0].Summary != want {
+		t.Errorf("Summary = %q, want %q", events[0].Summary, want)
+	}
+}
+
+// A day is 23 hours wide where DST starts, so a wall clock reading has to be
+// built with time.Date and never as midnight plus a duration.
+func TestFactSpanAcrossDSTStart(t *testing.T) {
+	loc, err := time.LoadLocation("Europe/Berlin")
+	if err != nil {
+		t.Skipf("no tzdata for Europe/Berlin: %v", err)
+	}
+	start, end, ok := FactSpan("calendar_event_20260329_Planerka", "Planerka @ 14:00-15:00", loc)
+	if !ok {
+		t.Fatal("FactSpan reported not ok")
+	}
+	if start.Hour() != 14 || start.Minute() != 0 {
+		t.Errorf("start = %s, want a 14:00 wall clock", start)
+	}
+	if end.Hour() != 15 {
+		t.Errorf("end = %s, want a 15:00 wall clock", end)
+	}
+}
+
 func TestReminderEventEmptyPayload(t *testing.T) {
 	e := ReminderEvent(3, time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC), "   ", 0)
 	if e.Summary != "напоминание" {