docs(6208): the remaining frontend comments drop milestone, step and rule citations

Components, stores, api clients, router, types and shared CSS now state what
each piece is and why, without the milestone that introduced it. diff.ts
names its consumers instead of the copy history; stores/notes and tasks state
their current scope instead of the cleanup that set it; theme.css states the
token practice instead of the sweep that established it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/stores/canonicalSystems.ts

@@ -6,7 +6,7 @@ import { useToastStore } from "@/stores/toast";
 import { apiErrorMessage } from "@/api/client";
 
 /**
- * The global area catalog (milestone 307). Shared by every project, so it is
+ * The global area catalog. Shared by every project, so it is
  * fetched ONCE per session rather than per project — the whole point of the
  * table is that it is the same list everywhere.
  */