style(repo-loading): refine overlay visuals and theming

Update the repository loading overlay and related UI styles to use
design tokens, improve contrast, and reduce visual bulk. Sizes,
spacing, radii, and icon dimensions were tuned to create a more
compact, consistent card. Backgrounds, grid textures, and shadows
were switched to CSS variables and color-mix; subtle backdrop-filter
and opacity tweaks improve legibility across themes.

- Replace hardcoded colors with theme variables and color-mix
- Reduce component dimensions and tighten spacing for a compact UI
- Add backdrop blur, adjust grid opacity, and simplify shadows

src/app.css

@@ -6475,6 +6475,7 @@ input:focus, textarea:focus, select:focus { box-shadow: 0 0 0 3px color-mix(in s
   display: grid;
   place-items: center;
   min-width: 0;
+  overflow: hidden;
   color: var(--color-primary);
   background: var(--color-surface-dim);
 }
@@ -6496,7 +6497,7 @@ input:focus, textarea:focus, select:focus { box-shadow: 0 0 0 3px color-mix(in s
   border: 1px solid color-mix(in srgb, var(--color-primary) 28%, var(--color-border));
   border-radius: 50%;
   background: var(--color-surface-solid);
-  box-shadow: 0 0 0 4px var(--color-surface-dim);
+  box-shadow: 0 0 0 2px var(--color-surface-dim);
 }
 .status-lane-head {
   display: flex;