Add proposal, design, specs and tasks for the fix-timezone-double-conversion
change. Documents the root cause (icsDateToJsDate returning fake UTC causing
double browser offset), the Intl.DateTimeFormat-based solution, and 40+
test scenarios covering DST, cross-timezone, half-hour offsets, and
round-trip conversions.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
