Add reactive date variables and date manipulation pipes

[jlowin]

The docs preview pipeline hardcodes JSON at build time, so using datetime.date.today() as a Calendar initial value freezes the preview to whatever date the docs were last built. This surfaced while updating Calendar docs to use the new value parameter.

Two related enhancements that would solve this and be generally useful:

Reactive date variables — built-in template variables like {{ $today }} or {{ $now }} that resolve at render time in the client, not at build time. These would let previews (and real apps) reference the current date without hardcoding.

Date manipulation pipes — template pipes for date arithmetic, e.g. {{ $today | addDays:7 }}, {{ $today | startOfMonth }}, {{ $today | addMonths:-1 }}. Combined with reactive date variables, this would cover common patterns like "default to next week" or "show the current month" without any Python-side date math.