Home mosaic, multi-select delete, offline banner and loading states

- Home puts the dashboards beside the flows: two equal-height columns,
  capped and scrollable, most recently worked on first. Each tile is a
  schematic footprint built from the stored widget placements.
- Flows and dashboards can be picked by long press or ctrl-click; the
  create button becomes a trash and one dialog covers the batch.
- The offline banner is drawn on the body so it centres on the viewport,
  and the live socket now releases the offline latch a stray 503 set.
- A boot spinner before React's first commit, a router pending screen for
  code-split pages, and skeletons where an empty list used to flash.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018tULRZJUkZsw7rMJ3h4xvu

frontend/src/components/Common/Loading.tsx

@@ -0,0 +1,21 @@
+import { Loader2 } from "lucide-react"
+
+/**
+ * The router's pending screen, shown while a page's code chunk is on its way.
+ *
+ * Deliberately only the spinner. The shell around it is already painted — this
+ * fills the content region alone — and the page behind it draws its own
+ * skeletons once its code arrives, so a second guess at that layout here would
+ * only be a shape to correct a moment later.
+ */
+export function PageLoading() {
+  return (
+    <output
+      aria-label="Loading"
+      data-testid="page-loading"
+      className="flex min-h-64 items-center justify-center"
+    >
+      <Loader2 className="size-6 animate-spin text-muted-foreground" />
+    </output>
+  )
+}