Use the brand loading mark instead of lucide's spinner

Copies index/frontend's FluksioLoader component and its
`.fluksio-loader-line` rule into the app (the shadcn duplication model, so
the CSS travels with the component) and swaps every Loader2 pending state
over: the router's pending screen, the flow dock's Run and Publish, the
dashboard editor's Publish, the overview toolbar's Publish-all and
LoadingButton. One loading mark across both shells.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K1moruzue2kTJd3uVisgNk

frontend/src/components/Common/Loading.tsx

@@ -1,4 +1,4 @@
-import { Loader2 } from "lucide-react"
+import { FluksioLoader } from "@/components/ui/fluksio-loader"
 
 /**
  * The router's pending screen, shown while a page's code chunk is on its way.
@@ -15,7 +15,7 @@ export function PageLoading() {
       data-testid="page-loading"
       className="flex min-h-64 items-center justify-center"
     >
-      <Loader2 className="size-6 animate-spin text-muted-foreground" />
+      <FluksioLoader className="size-6 text-muted-foreground" />
     </output>
   )
 }