feat(form): expose form values + autocomplete hooks for custom dependent components

[stmh]

Summary

Implements the four API extensions proposed in #29 so consumers can build custom dependent-autocomplete field components without forking the library.

• flowdrop:getFormValues context in ConfigForm.svelte — registered field components can read sibling form values to resolve dependencies.
• Registry branch in FormField.svelte — mirrors the existing branch in FormFieldLight.svelte, so custom components registered via fieldComponentRegistry get rendered in the full form too, with autocomplete and dependencies props forwarded.
• autocomplete / dependencies props forwarded in FormFieldLight.svelte — the registered-component branch now passes both through, matching the new FormField branch.
• FieldSchema.dependencies type added in form/types.ts — Record<string, string> mapping URL param → sibling field name.
• form/autocomplete subpath export — new src/lib/form/autocomplete.ts barrel re-exports FormAutocomplete so consumers can wrap it without reaching into private paths; corresponding entry added to package.json exports.

Together these are the minimum hooks needed for a downstream app to ship its own dependent-autocomplete field (e.g. an autocomplete whose query depends on the value of another field in the same form) using only public API.

Closes #29.

Test plan

• prettier --check on changed files (clean)
• svelte-check on touched files (no new errors; one pre-existing error in Playground.svelte is unrelated)
• Maintainer review of API surface
• Consumer smoke test: register a custom component that reads getContext('flowdrop:getFormValues') and renders FormAutocomplete with dependencies