feat: Add event creation, editing, and deletion

[tienou]

Summary

This PR adds full CRUD (Create, Read, Update, Delete) capabilities for calendar events directly from the week-planner-card — no need to leave the card or use external tools.

Features

• ➕ Create events: A discreet "+" button appears at the bottom of each day cell. Clicking it opens a creation dialog pre-filled with the selected day's date (rounded to the next hour). The form includes title, calendar selector (populated from configured calendars), start date/time, and optional end date/time.

• ✏️ Edit events: Clicking on any event opens an edit dialog directly (single click workflow). The form is pre-filled with the event's current data (title, calendar, start/end times). Uses reactive form state (_editFormData property with @input handlers) to prevent value loss during re-renders.

• 🗑️ Delete events: A delete button is available in the edit dialog, allowing users to remove events. Uses the calendar/event/delete WebSocket API with support for recurring events (recurrence_id + recurrence_range).

• 🔄 Smart update fallback: Event updates first try the native calendar/event/update WebSocket API. If the calendar integration doesn't support it (returns not_supported), it automatically falls back to a delete + recreate strategy using calendar/event/delete + calendar.create_event service call.

Technical details

• New reactive properties: _showCreateEventDialog, _showEditEventDialog, _editFormData
• New render methods: _renderCreateEventDialog(), _renderEditEventDialog()
• New handlers: _handleAddEventClick(), _handleCreateEvent(), _handleUpdateEvent(), _handleDeleteEvent(), _handleDeleteEventFromEdit(), _handleEditEventClick()
• Event creation uses this.hass.callService('calendar', 'create_event', ...)
• Event deletion uses this.hass.callWS({ type: 'calendar/event/delete', ... })
• Event update uses this.hass.callWS({ type: 'calendar/event/update', ... }) with fallback
• Styles added for the "+" button, creation/edit forms, and action buttons
• All dialogs auto-refresh events after successful operations via _updateEvents()

Screenshots

The "+" button is subtle (30% opacity) and becomes more visible on hover (80% opacity). The creation and edit dialogs use native HTML inputs for broad compatibility with Home Assistant.

Related issues

Closes #298

Test plan

• Each day cell displays a "+" button at the bottom
• Clicking "+" opens a creation dialog with the correct date pre-filled
• Creating an event refreshes the calendar view
• Clicking an existing event opens the edit dialog with pre-filled data
• Modifying an event and saving works (including calendars without native update support)
• Deleting an event from the edit dialog works
• Recurring events are handled properly (recurrence_id support)
• Multiple configured calendars appear in the calendar selector dropdown

🤖 Generated with Claude Code