Default sending domain per workspace and personal default From address

[mwoodworth]

Scope & Context

Workspaces often send email for one brand from a mailbox that has several "Send mail as" aliases, for example a shared Google Workspace mailbox with an alias per product domain. Twenty already reads these aliases and lets you pick one in the From field. This issue is about choosing which address the From field starts on.

Current behavior

The From field in the email composer always starts on the connected account's main address. This applies to new emails, replies and new workflow "Send Email" steps. Every email needs the right alias picked by hand, and a slip sends a customer an email from the wrong brand's address.

Expected behavior

• A workspace admin can set a default sending domain for the workspace (e.g. example.com). The From field then starts on the member's sendable address at that domain.
• Each member can optionally set a personal default From address that takes precedence over the domain.
• The rule applies to new emails, replies (without switching to a different connected account, so threading is preserved) and new workflow "Send Email" steps.
• The From field can still be changed on any email. With nothing configured, behavior is unchanged.

Technical inputs

• Two nullable columns: core.workspace.defaultSenderDomain (WORKSPACE permission) and core.userWorkspace.defaultFromHandle.
• One shared resolver in twenty-shared, used by the composer and workflow step creation.
• Sending and server-side sender validation stay as they are.

Implementation proposed in #28654.