feat(account): unified special folders

[wmontwe]

Contribution Summary

Linked Issue/Ticket: Resolves #9370 Resolves #9773

Fixes #6596, #9773, #8578. Makes #9571 obsolete. Partially addresses #6799 (unified folders only).

Description

Adds unified folders to the drawer: Inbox, Outbox, Drafts, Sent, Archive, Spam and Trash across all accounts. Each one resolves to every account's own special folder.

This also adds a new empty view to the message list to give users feedback if folders contain messages or have not been loaded yet.

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AI Disclosure

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• This contribution does not include any changes created or assisted by AI.
• This contribution includes changes assisted by AI.
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• This contribution includes changes created by AI.

Stack

• #11708 feat(account): decouple account management and storage from legacy Preferences
• #11744 feat unified special folders ← current

Commits to review

• a3a48b9 feat(search): resolve unified searches to each account's own folder
• 7ece044 feat(folder): remove the include in unified inbox setting
• 76fdd5b feat(drawer): add unified special folders
• 2befefb refactor(drawer): remove stale key
• 118f987 feat(message-list): sync unified folders on refresh
• 1863c02 feat(message-list): show empty state
• 9093169 fix(message-list): only allow pull to refresh for remote folders
• f1f8ac1 refactor(widget): describe the unified inbox as every account inbox

This PR includes earlier stacked changes because GitHub cannot use a branch from a fork as its base.

[github-actions]

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