refactor(app): spacing, radius and motion tokens

theme::space, theme::radius and theme::motion name the steps the UI is
drawn with. Literal gaps, corner radii and animation durations across ui/
now come from them, which folds a few near-misses onto the scale (10 and 14
point gaps onto 12, 10 point radii onto 12, 0.10 s hovers onto 0.12 s).
Page fades and toasts share one ease_out curve.

android/app/src/main/java/io/narl/protonstream/ui/theme/Shape.kt

@@ -29,7 +29,7 @@ internal val ProtonShapes = Shapes(
 /** The radius `pstr-app` draws everything at. */
 internal const val CORNER = 8f
 
-/** A badge over artwork — `ui/mod.rs` is the other place this number lives. */
+/** A badge over artwork — `theme::radius::BADGE` in pstr-app is the other place this number lives. */
 internal const val CORNER_TIGHT = 4f
 
 /** A bottom sheet, which has no desktop counterpart. */