feat(ui): use a warm burgundy for the last-word highlight in both themes

The gold/brown recent colour shared the tile's warm hue, so it could not separate from both the near-black glyph (when dark) and the tan tile (when light) at once. Switch --tile-recent to a burgundy #8c4a3c whose red hue stays distinct from both, in light and dark, and unify the value across all three theme blocks.

ui/src/app.css

@@ -29,7 +29,7 @@
   --tile-edge: #d8c190;
   --tile-text: #2a2113;
   --tile-pending: #f2cf73;
-  --tile-recent: #a8884a;
+  --tile-recent: #8c4a3c;
   --prem-tw: #e06a5b; /* triple word */
   --prem-dw: #efa6a0; /* double word + centre */
   --prem-tl: #4f8fd6; /* triple letter */
@@ -75,7 +75,7 @@
     --tile-edge: #b6a473;
     --tile-text: #20190d;
     --tile-pending: #d8b75e;
-    --tile-recent: #6c5a30;
+    --tile-recent: #8c4a3c;
     --prem-tw: #9c3f34; /* 3x word: a touch darker red */
     --prem-dw: #a8636b; /* 2x word: softer, pinker */
     --prem-tl: #2c527a; /* 3x letter: a touch darker blue */
@@ -106,9 +106,9 @@
   --tile-edge: #b6a473;
   --tile-text: #20190d;
   --tile-pending: #f0d98f;
-  /* Last-word highlight letter colour; matches the OS-dark value so the highlight reads the
-     same whether dark is chosen in Settings or via prefers-color-scheme. */
-  --tile-recent: #6c5a30;
+  /* Last-word highlight letter — a warm burgundy accent shared across themes; its red hue
+     stays distinct from both the near-black glyph and the warm tile, in light and dark. */
+  --tile-recent: #8c4a3c;
   --prem-tw: #9c3f34; /* 3x word: a touch darker red */
   --prem-dw: #a8636b; /* 2x word: softer, pinker */
   --prem-tl: #2c527a; /* 3x letter: a touch darker blue */