feat(ui): tint last-word letters for the recent highlight; lift dark bonus contrast

Dark theme: the 2x/3x bonus-square pairs were too close to tell apart. Soften the 2x squares (sky blue #4a779b, rose #a8636b) and deepen the 3x squares (#2c527a, #9c3f34) so each pair reads as two distinct steps. Light theme is unchanged.

Last-word highlight (both themes): stop tinting the tile background — the tile keeps its normal fill, and instead the placed letters (not the point values) are drawn in the recent-move colour. The opponent-just-moved flash now pulses the letter between its normal colour and the recent colour, with no background animation and no white peak.

Reconcile the explicit [data-theme=dark] --tile-recent with the OS-dark value so the highlight reads the same however dark is selected, and darken --tile-recent a step in every theme. Update docs/UI_DESIGN.md.

docs/UI_DESIGN.md

@@ -123,9 +123,11 @@ Login uses `Screen`.
   overlays the empty area below, so the layout doesn't resize/jank; other modals stay
   keyboard-aware (they size to the area above the keyboard).
 - **Highlights**: pending tiles use a slightly darker tile background (no outline). The
-  last completed word gets a dark tile background — static while it is the opponent's
-  turn (our word), and a 1 s flash when it is our turn (their word). While placing, only
-  the pending tiles are highlighted.
+  last completed word keeps the normal tile background; instead its letters — not the point
+  values — are drawn in the recent-move colour, in both themes. It is static while it is the
+  opponent's turn (our word), and a 1 s flash (the letter pulses between its normal colour
+  and the recent-move colour) when it is our turn (their word). While placing, only the
+  pending tiles are highlighted.
 - **Bonus-square labels** — a Settings choice (`boardlabels.ts`): `beginner` shows a
   split `3×` / `word` (localized слово/буква), `classic` a single `3W` / `3С`, `none`
   nothing. Default **beginner**.