🐛 fix: pass postcode to agent address lookup so search actually fires

[arturasmckwcz]

Summary

• useAgentAddressLookup never sent postcode to GET /agent/register/search, so the backend guard (!postcode) always short-circuited and returned empty data
• The street-based agent prepopulation during self-registration was completely broken — the lookup never ran regardless of street length
• Fix: accept addressPostcode in the hook, debounce it, include it in the query string, and require it (alongside street ≥ 3 chars) before enabling the request

Test plan

• Open the agent self-registration flow (/agent/register?token=...)
• Type a street with ≥ 3 chars and a valid postcode — confirm the match banner appears when the address corresponds to an existing agent
• Confirm dismissing the banner and completing the create-new-agent flow still works
• Confirm the join flow (clicking "Use this org") still routes correctly to the membership request

🤖 Generated with Claude Code