Add full VTODO/task management via Evolution Data Server Python scripts
(list, create, complete/uncomplete, delete) and integrate into the panel
with a Calendar/Tasks tab bar, task list view with priority indicators,
due date display, and a create task dialog.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CalendarService.loadEvents() replaces all cached events with only the
requested date window. The previous approach called loadEvents() on
every week navigation, causing a race with CalendarService's internal
5-minute auto-refresh (which uses narrow defaults of 31/14 days).

Key changes:
- Request wide range (180 days back, 60 days ahead) on initial load
- Navigate weeks via refreshView() which re-filters locally
- Detect when auto-refresh narrows events and re-request wide range
- Process cached events immediately on startup (before EDS is ready)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

The plugin was stuck loading because loadEvents() called
updateEventsFromService() synchronously before CalendarService
had fetched EDS data. Now Main.qml listens to onEventsChanged,
onLoadingChanged, and onAvailableChanged signals to process
events when they actually arrive. Added 15s safety timeout.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>