Follow the system style, add a compact mode and show the 1-9 keys

[geodro]

Junction called set_color_scheme(FORCE_DARK) and hardcoded #353433 / #1e1e1e / white / black in style.css. On a light desktop the window stayed dark but libadwaita flipped the foreground colors, so the app labels and the window controls became dark-on-dark. Fixes #96.

These are separable changes on one branch, and I am happy to split them into separate PRs if you'd prefer.

Follow the system style

• The color scheme defaults to default, so libadwaita follows the org.freedesktop.appearance portal: color scheme, accent color and contrast. GNOME, KDE Plasma and others implement it.
• A Style menu section (Follow System / Light / Dark) overrides it, stored in a new color-scheme key whose values match AdwColorScheme so they can be passed straight to set_color_scheme.
• Hardcoded colors become @popover_bg_color, @view_bg_color, @borders and @accent_*. @popover_bg_color is within a shade of the old #353433 in dark, and gives a sane light appearance.
• The focused tile is tinted with the accent color rather than a fixed alpha(white, 0.3), which was invisible on light backgrounds.
• .welcome button.link no longer forces white text, which was invisible on a light background.
• A prefers-contrast: more block thickens the entry border and focus ring.

One popover per menu button

Both MenuButtons pointed at a single shared PopoverMenu instance. A GtkPopover can only have one parent, so GTK reparented it and logged gtk_widget_set_parent criticals on every launch. On Plasma the visible symptoms were a stray arrow pointing at the hidden header button, and the menu opening by itself. Each button now uses menu-model: and gets its own correctly parented popover from the same GMenu. The criticals go from one per launch to zero.

This one is independent of the theming work and could land on its own.

Compact mode

• A Compact Mode menu toggle backed by a new compact key: icons 92 to 64, tiles 134 to 80, tighter padding, window 772x218 to 560x140, and the window corner radius 30px to 18px because 30px cuts into the first tile at that height.
• Compact tiles are icons only regardless of Show App Names, because a name ellipsizes to uselessness under an 80px tile. The name stays in the tooltip.

Show the 1-9 keys

• The first nine tiles show the digit that opens them. The shortcut already existed but was only discoverable from the Keyboard Shortcuts window, and compact mode hides names entirely.
• getButtonForKeyval now also accepts KP_0 to KP_9, so the numeric keypad works and not just the number row.

[sonnyp]

Hello,

Thank you, please split this into

1. One popover per menu button
2. Follow system theme

For now and we'll look at 1-9 and compact mode afterwards.

[geodro]

Done, split as requested:

1. One popover per menu button: Give each menu button its own popover #219
2. Follow the system style: Follow the system style #220

I'll keep this branch around for the compact mode and 1-9 keys work to revisit afterwards — let me know if you'd rather I close this PR in the meantime.