feat(hyprland): themed backdrop — misc:background_color from the palette (item 28c)

The compositor backdrop behind a missing or still-loading wallpaper
was hard black; on light themes that flash reads as a glitch (caught
by the 1080p capture harness on summer-day). Now the theme's base
color, so every wallpaper-less moment stays in-palette.

Also from the summer-day capture review: the authored whole-swap menu
+ island bar survive the 28b retunes as one coherent identity (the
slate menu deliberately mirrors the bar strip — an earlier "polarity"
concern retracted); nit filed for the undimmed placeholder in the two
summer rasi. Punch list updated in item 28.

Verified: V0; V1 — template-home rebuilt, hyprland.conf renders
background_color=rgb(1a1b26). Visual confirmation rides the next
capture run.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/hyprland.nix

@@ -377,6 +377,11 @@ in
         disable_hyprland_logo = lib.mkDefault true;
         disable_splash_rendering = lib.mkDefault true;
         force_default_wallpaper = lib.mkDefault 0;
+        # The backdrop when no wallpaper is painted (awww still starting,
+        # a slow first paint, or a wallpaper-less setup): the theme's base
+        # instead of compositor black — on light themes the black flash
+        # reads as a glitch (item 28c, seen in the capture harness).
+        background_color = rgb c.base;
       };
 
       # Rendered from ./keybinds.nix (the cheatsheet reads the same list),