refactor(frontend): simplify remote save pipeline

[aksOps]

Summary

• replace positional remote-save callbacks with a typed acknowledgement object
• extract bounded helpers for legacy indexing, canonical ordering, request preparation, conflict retry, and acknowledgement delivery
• migrate only the two App save callback sites
• add characterization for identity, ordering, conflict, retry, wire, and callback contracts

Sonar scope

G006 Batch 1 targets 11 existing frontend-production findings:

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AZ-8DLECETN2Y3sJmEsH
AZ-8DLECETN2Y3sJmEsK
AZ-8DLECETN2Y3sJmEsL
AZ-8DLECETN2Y3sJmEsM
AZ-8DLECETN2Y3sJmEsN
AZ-8DLECETN2Y3sJmEsO
AZ-8DLECETN2Y3sJmEsP
AZ-8DLECETN2Y3sJmEsQ
AZ-8DLECETN2Y3sJmEsR
AZ-8DLECETN2Y3sJmEsS

Expected OPEN delta after exact-main analysis: 11 fewer, with zero new issues.

Verification

• signed head: 9de6898e59118301ac8eedb691b6963083f098eb
• targeted Vitest: 201 passed
• full frontend: 923 passed
• frontend coverage: 98.29% statements, 95.31% branches, 98.74% functions, 99.46% lines
• build: passed
• Go race: 938 passed; aggregate coverage 97.8%, lowest package 95.6%
• Go vet/format, Actionlint, ShellCheck, TypeScript, and diff checks: passed
• normalized remote/App differential: zero unexpected semantic mismatches
• independent code review: APPROVE
• hosted run 30726489805: success in 10m53s
• PR Sonar analysis 31473840-aa02-4690-a18b-668520f18e79: exact head, quality gate OK
• new code: 99.3% coverage, 0.0% duplication, 0 issues, 0 accepted issues, 0 hotspots

No dependency, workflow, persistence-format, rendering, keyboard, or accessibility change.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
99.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud