Give the canvas dot grid the token colour React Flow actually reads

`flow.css` set `--xy-background-pattern-dots-color`, which nothing reads:
xyflow's `.react-flow__background-pattern.dots` reads
`--xy-background-pattern-color` and only falls back to the `-dots-color-default`
literal. The grid was therefore pinned to `#91919a` in both themes — and with no
`colorMode` prop the `.react-flow.dark` default never applies either, so it read
as a light-mode grid on a dark canvas.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016ZeGnqVsf5VHQqvz4HdUhN

frontend/src/components/Flow/flow.css

@@ -8,7 +8,7 @@
 
 .react-flow {
   --xy-background-color: var(--background);
-  --xy-background-pattern-dots-color: color-mix(
+  --xy-background-pattern-color: color-mix(
     in srgb,
     var(--muted-foreground) 30%,
     transparent
@@ -251,7 +251,7 @@
    */
   --xy-edge-stroke-width: 3;
   --edge-pulse-width: 4;
-  --xy-background-pattern-dots-color: color-mix(
+  --xy-background-pattern-color: color-mix(
     in srgb,
     var(--muted-foreground) 22%,
     transparent