Add address autocomplete for incomplete address searches

[k7lim]

Problem

Users can enter a street-only address and get no results until they add city/state context. The lookup flow should help users complete ambiguous or incomplete addresses instead of only failing after submission.

Desired behavior

• Suggest likely full addresses or locality completions as the user types.
• Help inputs missing city/state/ZIP move toward a geocodable address.
• Preserve the project privacy posture: do not persist exact typed addresses, exact coordinates, raw upstream responses, or lookup history.
• Keep provider keys server-side if the selected autocomplete provider requires one.
• Show a useful empty/error state when suggestions are unavailable or the input is too ambiguous.

Notes

This lines up with docs/roadmap.md under Address Lookup. Provider selection should account for cost, quota, coverage in Hawaii, accessibility, and whether terms allow a public demo.