feat: support local-only offline projections

[rdlabo]

Summary

• allow replica entities without a server ID for local-only projections
• keep the existing at-most-one server ID invariant
• preserve strict nullability checks when strictNullChecks is enabled, while supporting legacy consumers that disable it

Verification

• npm run lint
• npm run prebuild:kit
• npx ng test kit --watch=false (33 files, 418 tests)
• integrated type-check and app tests against tipsys offline feed work

Compatibility

This relaxes a runtime restriction and keeps existing server-backed replica schemas unchanged.

[netlify]

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[rdlabo]

レビュー完了

• 上長レビュー: APPROVED（Web/SQLite parity、transaction原子性、server-backed後方互換を確認）
• 第三者受け入れレビュー: APPROVED（local-only round-trip、serverId lookup、誤用rejectを独立確認）
• CI: 全check green
• ローカル検証: lint / kit production build / 422 tests / nonstrict type compile / diff-check pass

HEAD: 3811f29

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.