fix(mail): keep sender column tied to row folder

[KaannKara]

Summary

• Fixes Incorrect folder content when switching folders #1825 by choosing From/To values from each row's actual folder instead of the currently selected folder.
• Covers both normal message lists and Xapian search results so stale rows do not briefly redraw with the wrong sender/recipient field while switching into or out of Sent folders.
• Adds focused regression tests in separate test commit.

Validation

• npx tsc -p src/tsconfig.spec.json --noEmit
• npx eslint src/app/common/messagelist.spec.ts src/app/xapian/searchmessagedisplay.spec.ts
• npm run lint (passes with existing warnings)
• npm run build (passes with existing warnings)
• npm run policy (exits 0; reports existing historical commit-message warnings)

Karma browser specs could not be executed in this environment because Firefox is not installed and ChromeHeadless is not registered in this repo's Karma config.

AI disclosure

This contribution was prepared with AI assistance and manually reviewed before submission.

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