fix: unify mail printing

[GVodyanov]

Fix mail printing... again

This makes emails with multiple images not be all forced into one page

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[ChristophWurst]

Do you have before/after examples and can tell us what we should test?

Code looks … complicated

[ChristophWurst]

/backport to stable5.10

[GVodyanov]

Do you have before/after examples and can tell us what we should test?

Code looks … complicated

Yeah fair enough

Here's a file which the old version for me had trouble printing, it had an extra page and lots of white space:

Grazie da Italo!.eml

Here is a full changelog:

• Unifies the two separate print code paths (Ctrl/Cmd+P for a whole thread, and the "⋯ → Print" action for a single message) into one shared pipeline in src/util/printMessage.ts.
• Fixes: wrong message body could be printed for the "⋯ → Print" action in threads mixing plain-text and HTML messages, because the old code picked the DOM node via querySelectorAll(...)[threadIndex * 2], an index that only holds if every prior message is HTML.
• Fixes: printed header could be enlarged/misstyled by the email's own CSS (the old code inserted it into the same iframe as the email).
• Fixes: brief visible flash in the open message when printing a single message (old code inserted/removed content in the live, on-screen iframe).
• Fixes: Ctrl+P could throw/fail on a message with no "To" recipient (e.g. Bcc-only).
• Adds: Cc/Bcc are now shown in the printed header (previously omitted, or folded anonymously into a thread-wide participant list).
• Hardens security: printed content now renders in a sandboxed iframe (allow-same-origin allow-modals, no scripts) instead of an unsandboxed one; <script> tags and iframe-resizer markers are stripped as defense in depth.
• Cleanup now waits on the afterprint event (with a 60s fallback) instead of guessing with fixed setTimeouts.
• Native browser print (outside the app's own Ctrl+P handler) now hides app chrome (nav, message list, composer, attachments) via CSS.