Correct the flow.css header: the canvas does pass colorMode

The comment said the canvas follows the theme without a `colorMode` prop. It
does not — React Flow stamps that prop on the wrapper as a class and defaults
it to `light`, which collided with the app's own `.light` token scope and
re-themed the whole canvas. The prop landed in 4e1898a; this is its reason.

frontend/src/components/Flow/flow.css

@@ -2,8 +2,10 @@
  * React Flow, routed through the design tokens.
  *
  * Scoped to `.react-flow` rather than added to index.css, so the canvas needs no
- * tokens of its own and follows the theme in both modes without a `colorMode`
- * prop. See the root DESIGN-GUIDELINES.md → Shells.
+ * tokens of its own. The `colorMode` prop is passed all the same: React Flow
+ * stamps it on the wrapper as a class, and `.light` / `.dark` are the app's own
+ * token scopes — an unset one defaults to `light` and re-themes the whole canvas
+ * inside a dark shell. See the root DESIGN-GUIDELINES.md → Shells.
  */
 
 .react-flow {