Branding: provide a simple "light theme / no background image" preset

### Is your feature request related to a problem?

A common, arguably default, branding goal is: "make the login page white with my logo and remove the background image." Achieving this in 2026.5.5 requires an unexpectedly deep dive into PatternFly CSS variables, shadow DOM ::part() selectors, and workarounds for a bug where the Default flow background field won't stay empty and reverts to the mountain image. I had to override --ak-global--background-image, target ak-locale-select parts, and juggle prefers-color-scheme media queries — all of which is well beyond what should be necessary for a simple light theme.

### Describe the solution you'd like

A toggle or dropdown in System → Brands → Branding settings that offers a simple preset:

- "Default" (current dark/mountain background)
- "Light theme" (white background, authentik dark text, no background image)
- "Respect browser preference" (switches between the two above based on prefers-color-scheme)

This would set the underlying CSS variables internally and avoid exposing users to the shadow DOM complexity. A "No background image" checkbox would also solve the regression where the Default flow background field won't clear without workarounds.

### Describe alternatives that you've considered

The current Custom CSS field is powerful for advanced users, but it's the only path for a basic request. It also breaks across upgrades because selectors change.

### Additional context

This is probably the most common branding request from new adopters. A friction-free path here would make Authentik significantly more approachable for teams evaluating it against commercial alternatives that offer one-click white-labeling.