Post-PR full remediation: stabilization and campaign safety

[ajbergh]

Summary

This PR implements the post-PR remediation program and establishes a hardened, green automated integration baseline for MailMerge Go.

Completed

• Stabilized CI and synchronized the frontend lockfile.
• Migrated to golangci-lint v2 and resolved findings without weakening the gate.
• Added blocking Go vulnerability and production npm security scans.
• Added backend/frontend coverage evidence and CycloneDX SBOMs.
• Added a manual release-candidate workflow, release manifests, comprehensive checksums, and optional Authenticode signing hooks.
• Persisted immutable campaign snapshots with durable IDs, parent/child retry lineage, run numbers, and restart-safe retry-by-campaign-ID.
• Added campaign-history APIs and UI for review, retry, delete-one, and clear-all.
• Added accessible test-send and preflight dialogs, rendered preview, draft-only mode, Windows-aware attachment deduplication, and an application error boundary.
• Hardened Outlook COM initialization, failure classification, capability reporting, and worker shutdown.
• Added Windows root-package tests for persistence, restart-safe retry, immutable snapshots, draft-mode persistence, and headless execution.
• Completed a final documentation pass across README, changelog, contributor guidance, phase status, release checklist, and release-evidence documentation.

Automated validation

The permanent CI workflow validates:

1. Go formatting, vet, race tests with coverage, and golangci-lint.
2. Frontend install, tests with coverage, and production build.
3. Reachable Go vulnerability scan and production npm audit.
4. Go and frontend CycloneDX SBOM generation.
5. Windows Wails build, root-package tests, and executable artifact upload.

Merge and release decision

This PR is ready for squash merge after all five jobs pass on the final documentation head.

Publishing a versioned release remains separately gated by dev_docs/release-checklist.md on a clean supported Windows system with classic Outlook and a configured mail account. Sender-account selection and Microsoft Graph delivery remain intentionally deferred and are not advertised as supported capabilities.