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/icssub/icssub.go

@@ -5,6 +5,7 @@
 package icssub
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"io"
@@ -14,6 +15,8 @@ import (
 	"time"
 
 	ical "github.com/emersion/go-ical"
+
+	"github.com/yourusername/caldav-server/internal/icalfix"
 )
 
 // DefaultTTL is how long a fetched calendar is cached before being
@@ -103,7 +106,14 @@ func (c *Cache) fetch(rawURL string) (*ical.Calendar, error) {
 		return nil, fmt.Errorf("fetching calendar: unexpected status %s", resp.Status)
 	}
 
-	cal, err := ical.NewDecoder(io.LimitReader(resp.Body, maxBodySize)).Decode()
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxBodySize))
+	if err != nil {
+		return nil, fmt.Errorf("reading calendar: %w", err)
+	}
+	// Some remote feeds (Outlook/Exchange-backed ones especially) use
+	// Windows timezone names instead of IANA ones, which go-ical can't
+	// resolve — fix those up before decoding (see internal/icalfix).
+	cal, err := ical.NewDecoder(bytes.NewReader(icalfix.NormalizeTimeZones(body))).Decode()
 	if err != nil {
 		return nil, fmt.Errorf("parsing calendar: %w", err)
 	}