refactor(frontend): simplify outbox persistence

[aksOps]

Summary

• simplify outbox delivery and retry state transitions without changing observable behavior
• isolate store source selection, generation validation, and snapshot parsing
• add regression coverage for failure ordering, lock rejection, malformed storage precedence, and cancellation timing

Sonar scope

Resolves the five Batch 2 maintainability findings:

• AZ-94_yl_sm3M2mrl-VN
• AZ-8DLDwETN2Y3sJmEsC
• AZ-8DLDwETN2Y3sJmEsD
• AZ-8DLDwETN2Y3sJmEsE
• AZ-8DLEmETN2Y3sJmEsV

Verification

• targeted Vitest: 179 passed
• full frontend suite: 929 passed
• frontend coverage: 98.30% statements, 95.29% branches, 98.75% functions, 99.46% lines
• Go race suite: 938 passed across 6 packages
• Go aggregate coverage: 97.8%
• build, TypeScript, vet, format, Actionlint, ShellCheck, and diff gates passed
• baseline/candidate storage fixture and cancellation traces matched exactly

[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