Align Calendar range selection with shadcn/ui mode prop

[maxffarrell]

Description

shadcn/ui exposes range selection as part of the Calendar component API instead of requiring a separate range calendar component for the common docs path.

Upstream citations:

• shadcn/ui describes Calendar as selecting "a date or a range of dates" in the component docs frontmatter: https://github.com/shadcn-ui/ui/blob/main/apps/v4/content/docs/components/base/calendar.mdx#L1-L8
• shadcn/ui documents Range Calendar as a Calendar example and says to use the mode="range" prop for range selection: https://github.com/shadcn-ui/ui/blob/main/apps/v4/content/docs/components/base/calendar.mdx#L148-L156
• Public docs show the same guidance: https://ui.shadcn.com/docs/components/calendar#range-calendar

Proposed outcome

• Allow <Calendar mode="range" /> in shadcn-svelte.
• Preserve existing <RangeCalendar /> usage for projects that already use it.
• Update docs so new range-calendar usage points to Calendar with the mode prop.
• Keep generated registry output complete so installing Calendar includes the range implementation needed by mode="range".

Related PR

#2732