refactor(toggle-group): use Svelte 5 createContext for type-safe context

[braden-w]

This replaces the manual setContext/getContext wrapper functions with Svelte 5's createContext factory in the toggle-group component.

The new pattern from the Svelte 5 docs provides automatic type safety without requiring string keys or explicit type annotations:

// Before (10 lines)
import { getContext, setContext } from "svelte";
export function setToggleGroupCtx(props: ToggleVariants) {
    setContext("toggleGroup", props);
}
export function getToggleGroupCtx() {
    return getContext<ToggleVariants>("toggleGroup");
}

// After (2 lines)
import { createContext } from "svelte";
export const [getToggleGroupCtx, setToggleGroupCtx] = createContext<ToggleVariants>();

The consumer (toggle-group-item.svelte) doesn't require any changes since the getToggleGroupCtx() API remains identical.

This is a pure refactor with no behavioral changes.

[changeset-bot]

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[huntabyte]

Thank you!