feat: add optional conversation reading pane

[YunQue0912]

Summary

Adds an optional Gmail-style full conversation reading pane while preserving flat-message and expandable-list modes.

Closes #316.

Changes

• Adds three persisted conversation display modes: off, expandable thread rows, and full reading pane.
• Loads complete conversations across folders, including sent replies, with lazy message-body loading and animated expand/collapse.
• Supports thread-wide read, archive, move, spam, snooze, delete, reply, and forward actions.
• Handles deep links, notifications, cached-thread revalidation, single-message fallback, and read-state rollback.
• Keeps the three conversation mode controls in one row across viewport sizes.

Testing

• Backend: npm test (706 tests)
• Backend: npm run lint
• Frontend: npm test (1449 tests)
• Frontend: npm run lint
• Frontend: npm run build
• git diff --check upstream/main...HEAD

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