Fix zone config modal crash

[mistercrunch]

Fixes #1759.

Summary

• Avoid rendering zone config/delete modals for every zone unless opened.
• Stabilize the zone data passed into the config modal and narrow form initialization dependencies.
• Derive zone node data typing from the canonical BoardObject union to reduce drift.
• Add regression coverage for opening the zone config modal, preserving edits across parent rerenders, and keeping the modal component unmounted until opened.

Checks

• ✅ pnpm i
• ⚠️ pnpm check — earlier run passed typecheck, Biome lint, and short-id checks; failed at check:multitenancy-boundaries due unrelated pre-existing baseline violations in daemon files.
• ✅ pnpm --filter agor-ui test -- src/components/SessionCanvas/canvas/BoardObjectNodes.test.tsx src/components/SessionCanvas/canvas/ZoneConfigModal.test.tsx
• ✅ pnpm --filter @agor-live/client build (needed after rebasing onto main so generated client types include newer board fields)
• ✅ pnpm --filter agor-ui typecheck
• ✅ pnpm exec biome check apps/agor-ui/src/components/SessionCanvas/canvas/BoardObjectNodes.tsx apps/agor-ui/src/components/SessionCanvas/canvas/ZoneConfigModal.tsx apps/agor-ui/src/components/SessionCanvas/canvas/BoardObjectNodes.test.tsx apps/agor-ui/src/components/SessionCanvas/canvas/ZoneConfigModal.test.tsx

Notes

• Rebasing onto current main resolved the merge conflicts by preserving the newer zone z-order/font-size work and layering this modal-mount fix on top.
• AI follow-up review passed; PR is labeled ai-reviewed-gpt-5-5.