fix(mail): open next message after delete

[keilogic]

Fixes #708.

Summary

• Route the single-message delete toolbar action through the same list-aware delete flow used by selected message deletes.
• When the deleted message is currently open and "Keep preview pane open" is enabled, select and open the next visible message after the removed message IDs, falling back to the previous visible message at the end of the list.
• Keep the existing close-the-viewer behavior when the preview pane is not configured to stay open or no visible messages remain.
• Add focused coverage for the row helper that chooses the next surviving message.

Testing

• npm ci
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npx eslint src/app/app.component.ts src/app/common/messagedisplay.ts src/app/mailviewer/rmm7messageactions.ts src/app/canvastable/canvastable.spec.ts
• npx ng test --include=src/app/canvastable/canvastable.spec.ts --watch=false --browsers=FirefoxHeadless (TOTAL: 5 SUCCESS; Firefox printed a post-success disconnect line but the command exited 0)
• npm run lint -- --quiet
• npm run policy
• npx ng test --watch=false --browsers=FirefoxHeadless (TOTAL: 249 SUCCESS, 2 skipped)
• node src/build/pre-build.js && npx ng build --configuration production --base-href=/app/ runbox7 && node src/build/post-build.js (build hash 9f88e81dcf7769f9, with existing Angular/CommonJS warnings)
• git diff --check
• git diff HEAD~1..HEAD --check
• git show --stat --oneline --check HEAD

AI disclosure

Codex assisted with issue triage, implementation, tests, and validation for src/app/app.component.ts, src/app/common/messagedisplay.ts, src/app/mailviewer/rmm7messageactions.ts, and src/app/canvastable/canvastable.spec.ts.