# Specification: Proper Timezone Handling & Recurring Events

## Overview
Upgrade the SilverBullet iCalendar plug to provide accurate, DST-aware timezone resolution and full support for recurring events (RRULE expansion). This replaces manual hour shifting with an automated, reliable system using IANA timezone standards and the `Intl` API.

## Functional Requirements
- **IANA Timezone Mapping:** Implement a comprehensive mapping of 139 Windows timezone names to IANA identifiers using Unicode CLDR data.
- **DST-Aware Offsets:** Calculate UTC offsets at runtime for specific event dates using the built-in `Intl.DateTimeFormat` API, ensuring accuracy during Daylight Saving Time transitions.
- **Robust Date Extraction:** Correct the wall-clock extraction logic to prevent "double-shifting" of event times.
- **Recurring Event Expansion:**
    - Integrate the `rrule` library to expand recurring events into individual occurrences.
    - Support `EXDATE` for excluding specific instances of a recurring series.
    - Implement a configurable `syncWindowDays` (default: 365) to limit the expansion range.
- **Advanced Filtering:**
    - Filter out "CANCELLED" events based on the iCalendar status field.
    - (Optional) Add `includeTransparent` and `includeDeclined` per-source flags.
- **Error Handling & Fallbacks:**
    - If a timezone is unrecognized, fallback to UTC and append a warning to the event's description.
- **Configuration Cleanup:**
    - Remove the redundant `tzShift` / `hourShift` parameters.
    - Add `syncWindowDays` global config.

## Non-Functional Requirements
- **Self-Contained:** Maintain the plug as a Deno-compatible project using `esm.sh` or `deno.json` imports.
- **Performance:** Ensure efficient expansion of recurrences, even for busy calendars.

## Acceptance Criteria
1. Events from various providers (Google, O365, Nextcloud) appear at the correct local time in SilverBullet, regardless of DST.
2. All occurrences of a weekly recurring event within the sync window are indexed.
3. Excluded dates (`EXDATE`) are correctly omitted from the index.
4. Cancelled events are not indexed.
5. The manual `tzShift` configuration is no longer required for correct time display.

## Out of Scope
- Full CalDAV synchronization (this remains a read-only `.ics` fetcher).
- UI for managing individual recurring instances (handled via SilverBullet queries).