Guard against cancelled existing events causing Invalid start time

[xiongyindong]

Summary

This patch addresses the recurring Invalid start time failure reported in #315 when syncing updates against existing Google events that are already cancelled.

What changed

• In Code.gs (startSync), filter out existing calendar events with status === "cancelled" immediately after fetch (with a log count).
• In Helpers.gs (processEvent), add a second safety guard: if a matched existing event is cancelled, skip Calendar.Events.update and log the skip.

Why this differs from existing fix attempts

There is already a focused filter fix in #526. This PR keeps that idea and adds a second update-path guard to handle edge cases where cancelled events can still appear during update flow.

Scope

• Minimal change, no behavior changes outside cancelled-event handling.
• No config changes.

Validation

• Code-path review against issue reproduction context.
• Logs now explicitly show when cancelled existing events are filtered/skipped.