Use a transparent branding asset for Outlook-safe share headers

[BionicCode]

Quick checks

• I searched existing issues first.
• For setup or policy problems, I checked the admin documentation linked above.

What happened?

Context

This is a follow-up to the compact default share-template work tracked in:

• [Community vote] Make the default share block more compact and mobile-friendly #12
• Fix Rights row rendering across email clients NC_Connector_for_Thunderbird#50
• Fix Rights icons and non-breaking FileLink values in Outlook Classic NC_Connector_for_Outlook#28

The revised template has been tested in Thunderbird, new Outlook, and Outlook Classic. The remaining header issue is caused by the opaque branding image and the header cell both providing their own blue background.

Problem

The current NC Connector header image includes a solid blue background.

The containing table cell also defines the blue branding background. This means the final header color depends on two separate rendered elements:

1. the background embedded in the image;
2. the background color of the surrounding table cell.

In Outlook Classic, these can render slightly differently or expose visible edges around the image. Scaling, image handling, and the Word-based HTML renderer can make the image background appear distinct from the surrounding header area.

The header should instead have one authoritative background-color source. Much like it is correctly rendered in Thunderbird and Outlook (new):

A logo with a transparent background could solve this problem.

Steps to reproduce

Render the default template in Outlook Classic.

Expected behavior

The header should have one authoritative background-color source for a harmonized visual appeal.
The logo background should blend seamlessly with the surrounding message background in Outlook Classic and other supported mail clients.

Proposed solution

• Add a logo asset with a transparent background.
• Replace the currently embedded opaque logo with the transparent variant.
• Preserve the existing logo dimensions, aspect ratio, alternative text and placement.
• Do not introduce client-specific background colours merely to compensate for the opaque image.

Acceptance criteria

• The logo asset used by the generated HTML has a transparent background.
• No rectangular background or colour mismatch is visible around the logo in Outlook Classic.
• The logo remains clearly visible and correctly proportioned.
• The existing layout and spacing are unchanged.
• Rendering remains correct in the other supported mail clients.
• Any embedded or Base64-encoded copy of the old logo is replaced as well.

NC Connector Backend version

1.2.2

Nextcloud version

Nextcloud Hub 26 Spring (34.0.1)

PHP version

8.3.32

License mode

Community

Affected area

Sharing policies / templates

Logs or screenshots