Feature Request: Add Preview function to account templates

[RealRaven2000]

Goal:

Provide a way for users to preview the behavior of their account templates in a composer environment before sending, with minimal setup and high fidelity.

Requirements:

• Preview button in the template settings.
• Open a dedicated preview window or pane showing the template applied to an example email.
• Use the an existing email in Thunderbird as the source for the preview when choosing Reply / Forward.
• Provide a Refresh button to reload the template
• Ensure templates render correctly with account/identity context.
• Preview shows a simplified version of the composer, disables sending emails, and clearly indicates that the information in this window is temporary.

[RealRaven2000]

Initial Implementation Goals

• as first implementation use an existing composer window / edit control to build the preview
• as the "follow up target" use the currently selected (single!) mail in the first 3pane tab. On clicking preview on a reply / forward tab for the first time in a session, always display a warning/info:

Please select any email from the folder in the first Thunderbird tab to reply/forward — currently [Inbox].
This preview cannot be saved or sent, but can be edited to test the template behavior in Composer.
[OK] [Cancel]

• make sure that the window looks sufficiently different and block sending with an alert

Preview only. If you want to use the edited mail, please copy it into a fresh composer window.