feat: add login_box_opacity config option

[jkinum]

Summary

• Adds login_box_opacity (float, 0.0–1.0, default 1.0) to the [background] section of regreet.toml
• When set below 1.0, the login box becomes translucent and the wallpaper shows through
• Default of 1.0 preserves existing fully-opaque behaviour — no breaking change

Why CSS alone can't do this

GTK4 CSS opacity on a GtkFrame is not respected at the theme rendering level; the frame's background is painted before CSS compositing occurs. Widget::set_opacity() in Rust is honoured by GTK4 and achieves the intended effect.

Implementation

File	Change
src/config.rs	login_box_opacity: f64 added to Background struct; Config::get_login_box_opacity() added
src/gui/templates.rs	Login GtkFrame named login_frame for direct access
src/gui/component.rs	login_frame.set_opacity() called during init with config value
regreet.sample.toml	login_box_opacity documented with note on why CSS can't do this

Example config

[background]
path = "/usr/share/backgrounds/greeter.jpg"
fit = "Contain"
login_box_opacity = 0.82   # wallpaper visible through login box

[rharish101]

Hi, can you instead try this CSS with the ReGreet on the main branch?

window > overlay > frame:nth-child(2) {
  opacity: 0.2;
}

You will have to edit your config for greetd/Wayland compositor (the one that launches ReGreet) to run regreet --style /path/to/your/style.css (replace /path/to/your/style.css with the path to the above CSS). You can also just run ReGreet in demo mode with this CSS.

I tried this on my system (with Arch Linux), and it worked for me. If it doesn't work for you, maybe we have to instead debug why it doesn't work for you, which means that it wouldn't be a feature PR, rather a bugfix PR.

As for the other things in this PR:

1. The video background feature: that should be its own PR. EDIT: I added this myself based on your latest commit, thanks a lot for the code.
2. Fedora packaging: I'd like to keep out the packaging stuff, since that's meant for distro maintainers (so you should send your code to them instead). I want to keep upstream only focused on the core code + the bare minimum needed to help distro maintainers.