fix(ui): readable light mode over the board backdrop

Body text washed out in light mode — the secondary/tertiary text tokens were
too light against the bright sandstone board image. Darken light --text-dim and
--text-faint, and heavily strengthen the light-theme scrim so the board drops
to a faint texture behind content. Dark mode unchanged.

src/styles/tokens.css

@@ -78,8 +78,10 @@
   --border-strong: rgba(2, 6, 23, 0.18);
 
   --text: #0e1726;
-  --text-dim: #475467;
-  --text-faint: #6b7689;
+  /* Darker secondary/tertiary text — the body sits over the (light) board image,
+     so these need more contrast than on a flat page to stay readable. */
+  --text-dim: #2c3a4d;
+  --text-faint: #46546a;
 
   /* Accent — same electric cyan→magenta as dark, dropped in value for contrast
      on the light sandstone board. */