[C2] Enable logo changes with custom styles

[ywarnier]

Still missing in C2.
The logo should go in the custom-CSS directory. More details in private issue.

Refs BT#21621

[ywarnier]

For now (with #5614) the logo has to be added manually in var/themes/[your-theme]/images/header-logo.png in 180x40 pixels, but it works. We're still in alpha, so that's cool for now.

[ywarnier]

This issue evolves a little bit: it should be possible to upload 4 files (all optional):

• platform logo in SVG (var/themes/[the-theme]/images/header-logo.svg)
• platform logo in PNG (var/themes/[the-theme]/images/header-logo.png)
• email logo in SVG (var/themes/[the-theme]/images/email-logo.svg)
• email logo in PNG (var/themes/[the-theme]/images/email-logo.png)

The SVG file is used in priority, if it is present.

The header logo in PNG must be max 190px wide and 60px high.

The recommended email logo width is 540px as per Chamilo 1.11 ($_configuration['email_logo']) (and 170px height if we respect the proportions of header-logo.png). In Chamilo 2, as soon as there is a file in there, it is served as e-mail logo.

[christianbeeznest]

• 4 optional logos per theme in var/themes//images/: header-logo.svg|png, email-logo.svg|png (SVG takes priority).

• Rules & endpoints: header PNG ≤ 190×60; email PNG ~540px (advised). POST /themes/{slug}/logos, DELETE /themes/{slug}/logos/{type}, GET /themes/{name}/{path}?strict=1&t=… (creates /images, sanitizes SVG, cache-bust).

• UI: AdminBranding shows effective + per-file previews and Remove, with fallback SVG→PNG→default; PlatformLogo uses the same fallback and default logo.

All included in PR: #6602

Thanks for confirmation.