refactor(e10): move the platform components into components/ui/ and enforce the boundary

The app HAS a platform layer -- AnchoredPopover (22 importers, with
arrows/typeahead/Escape/anchor-flip), QueryState (21), Modal (17),
PageShell, DataTable, Tabs, Tooltip and ~26 more -- but it sat flat among
~160 feature components with nothing marking it as one. Several of those
files describe themselves as 'the one place' or 'canonical' and were
still bypassed by hand-rolled markup, because a layer nobody can see is a
layer nobody uses; three separate inventory passes of this codebase
misread it the same way.

Moves the 33 domain-free modules into components/ui/, rewrites the
imports in 89 files, and makes the boundary a lint rule rather than a
convention: inside ui/ a one-level-up import (a feature component) and
any lib/api slice are errors. The rule was verified to fire on both and
to leave generic lib/ helpers alone. README.md states which side of the
line a component belongs on and why.

No behaviour change: 3x tsc, eslint, knip, prettier and 687 vitest tests
all green; no backend file touched.

frontend/src/components/ui/ItemCount.tsx

@@ -0,0 +1,35 @@
+import { useTranslation } from "react-i18next";
+import { Layers } from "lucide-react";
+
+// At a million and above we stop printing the exact figure — six digits is the readable ceiling, so
+// the pill shows "1M+" and the tooltip carries the precise number.
+const OVERFLOW_AT = 1_000_000;
+
+// A compact, FIXED-WIDTH count pill for a toolbar: the filtered item count in the same glass pill as
+// the filter dropdowns, minus the chevron and the interactivity. The fixed width is the point — a
+// changing number must never shove the controls beside it, so the box is sized to the widest value
+// ("999,999 items") and the content is centred within it. A count of a million or more collapses to
+// "1M+", with the exact figure in the title tooltip. Deliberately generic ("item(s)", dynamic plural)
+// so any surface can reuse it for whatever it counts.
+export default function ItemCount({ count }: { count?: number }) {
+  const { t } = useTranslation();
+  // `count` is undefined only on a first, un-cached load — keep the fixed box (nothing shifts), just
+  // show the icon until the number arrives.
+  const label =
+    count == null
+      ? ""
+      : t("common.itemCount", {
+          count,
+          formattedCount: count >= OVERFLOW_AT ? "1M+" : count.toLocaleString(),
+        });
+  return (
+    <span
+      data-testid="item-count"
+      title={count?.toLocaleString()}
+      className="glass-card inline-flex shrink-0 items-center justify-center gap-1.5 rounded-full border border-border px-3 py-1 text-sm text-muted min-w-[9rem] whitespace-nowrap"
+    >
+      <Layers className="h-3.5 w-3.5 shrink-0 text-muted" aria-hidden="true" />
+      <span className="tabular-nums">{label}</span>
+    </span>
+  );
+}