feat: add message pane aware context menu

[dcoffin88]

Summary

This PR adds a context menu to the message pane, providing common actions within email content. Originally for #301 but also improves main app.

Changes

• Added a message pane context menu.
• Copy copies the current text selection from the message pane, matching standard text-selection behaviour, and is disabled when nothing is selected.
• Select All selects the entire message, including HTML emails rendered inside iframes.
• Find opens an in-app search dialog to search within the currently open message, automatically scrolling to and selecting matches.
• Context menu handling ensures a stale text selection elsewhere in the application does not prevent the message pane context menu from opening.

Testing

Tested on my production environment using both plain-text and HTML emails.

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