Add RRuleEditor composite widget (recurrence editor for calendar / scheduling)

[prakashUXtech]

Context

Recurrence editing in operator-surface forms typically needs a UI that wraps the RFC 5545 RRULE format. Today there's no composite widget for this — calendar event drawers, recurring-task editors, scheduled-pocket triggers all need to roll their own.

Surfaced during paw-enterprise#220 (calendar event drawer). Recurrence was scoped down to a plain dropdown (none / daily / weekly / monthly / custom) because composing the full editor inline would have ballooned the PR.

What to add

src/lib/widgets/composite/RRuleEditor.svelte — composes existing primitives into a usable RRULE builder:

• Frequency: Segmented control (None / Daily / Weekly / Monthly / Yearly / Custom)
• Interval: NumberInput (Every N days / weeks / etc.)
• By weekday (when frequency = Weekly): toggle group of S M T W T F S
• By monthday (when frequency = Monthly): dropdown for "Day N of month" OR "Nth weekday of month"
• Terminator: Segmented (Never / Until date / After N occurrences)
• End date (when terminator = Until): DatePicker
• Count (when terminator = After): NumberInput
• Preview: human-readable summary ("Every 2 weeks on Mon, Wed, Fri until Dec 31")
• Custom mode: bypass the form and accept a raw RRULE string with validation

API sketch

<script>
  let { value = $bindable(''),         // RRULE string (RFC 5545)
        startDate = new Date(),         // anchor for the recurrence
        timezone = 'browser',
        showPreview = true,
        disabled = false } = $props();
</script>

Internally uses rrule npm package or dateutil-equivalent for parsing/generation. The bindable value is always a normalized RRULE string consumable by any RFC 5545-compatible backend (gcal, outlook, ical, pocketpaw#1132's Calendar module).

Design considerations

• Composes existing primitives only (Segmented, NumberInput, DatePicker, Select). Don't reinvent.
• Preview line gives non-technical operators confidence in what they've configured
• Keyboard navigation across the form fields
• Accessibility: each section has a clear label; preview is an aria-live region
• Reject invalid combinations with inline error messages

Acceptance

• Component file at src/lib/widgets/composite/RRuleEditor.svelte
• Test file covering: round-trip parse/generate, common patterns (daily-for-30-days, every-other-week-on-MWF, monthly-on-15th, yearly-on-anniversary), invalid input handling
• Exported from src/lib/widgets/composite/index.ts
• Preview line renders for all supported patterns

References

• Triggering PR: qbtrix/paw-enterprise#220
• Adjacent: ChipInput (filed separately — could be used for byweekday selection)
• RFC: https://datatracker.ietf.org/doc/html/rfc5545#section-3.3.10