fix: stop unified-mailbox from mutating client-returned email objects

[hildebrandttk]

Summary

The cross-account fan-out helpers in lib/unified-mailbox.ts stamp the source
account info (accountId, accountLabel, sourceClientAccountId, sourceAccountId,
sourceFolder) onto every email before merging. main does this by MUTATING each
email object in place:

  email.accountId = account.accountId;
  email.accountLabel = account.accountLabel;
  ... (3 more fields)

Those objects are shared references handed back by the per-account JMAP client.
Mutating them in place could surprise any caller that retained them and corrupt
an account-state snapshot. The cross-account "All accounts" feature (a29c33b)
widened the bug: it carried the original two mutation sites forward and added a
third in the shared/group fan-out, so there are now three sites:

• lib/unified-mailbox.ts fetchUnifiedEmails
• lib/unified-mailbox.ts fanOutUnifiedQuery
• lib/unified-mailbox.ts fanOutCrossQuery (new, from the cross-account feature)

Changes

• lib/unified-mailbox.ts: at all THREE fan-out sites, decorate shallow copies
  instead of mutating the client's objects:

  const decorated = result.emails.map((email) => ({
    ...email,
    accountId: account.accountId,
    accountLabel: account.accountLabel,
    sourceClientAccountId: account.clientAccountId,
    sourceAccountId: account.jmapAccountId,
    sourceFolder: resolveSourceFolderName(email, account.mailboxes),
  }));
  mergedEmails = mergedEmails.concat(decorated);
  

  All five stamped fields are preserved; the merged result is identical, only the
  client's original objects are now left untouched.

• lib/tests/unified-mailbox.test.ts: flips the existing CHARACTERISATION test
  ("mutates the source email objects in place") into "does NOT mutate the source
  email objects (decorates copies)" - asserts the returned email carries the
  account info while the input object has neither accountId nor accountLabel.

Related Issues

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor / code quality improvement
• Chore / dependency update / CI change

Checklist

• I have read the Contributing Guide
• My code follows the project's code style and conventions
• I have run npm run typecheck && npm run lint and there are no errors
• The build passes (npm run build)
• I have tested my changes locally
• [] I have added or updated documentation if needed
• I have updated translations (locales/) if my changes affect user-facing text
• I have included screenshots or a screen recording for UI changes

Screenshots / Demo

Notes for Reviewers