feat(ui): lighten the light-theme last-word highlight to a brighter burgundy

At #8c4a3c the highlight blended into the lighter board in the light theme. Give the light theme a lighter burgundy #9c5849 while the dark theme keeps #8c4a3c — the two are tuned per theme because perceived contrast depends on the surrounding board tone.

ui/src/app.css

@@ -29,7 +29,9 @@
   --tile-edge: #d8c190;
   --tile-text: #2a2113;
   --tile-pending: #f2cf73;
-  --tile-recent: #8c4a3c;
+  /* Last-word highlight letter — a lighter burgundy than the dark theme on purpose: against
+     the lighter tile the perceived contrast needs it, so the two are tuned per theme. */
+  --tile-recent: #9c5849;
   --prem-tw: #e06a5b; /* triple word */
   --prem-dw: #efa6a0; /* double word + centre */
   --prem-tl: #4f8fd6; /* triple letter */
@@ -106,8 +108,9 @@
   --tile-edge: #b6a473;
   --tile-text: #20190d;
   --tile-pending: #f0d98f;
-  /* Last-word highlight letter — a warm burgundy accent shared across themes; its red hue
+  /* Last-word highlight letter — a warm burgundy whose red hue stays distinct from both the
-     stays distinct from both the near-black glyph and the warm tile, in light and dark. */
+     near-black glyph and the warm tile. The light theme uses a lighter burgundy (tuned per
+     theme; perceived contrast depends on the surrounding board). */
   --tile-recent: #8c4a3c;
   --prem-tw: #9c3f34; /* 3x word: a touch darker red */
   --prem-dw: #a8636b; /* 2x word: softer, pinker */