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.dom.test.tsx

@@ -0,0 +1,45 @@
+// @vitest-environment jsdom
+//
+// Contract of the fixed-width count pill. Uses the REAL i18n instance (not a key-passthrough) so the
+// plural selection (itemCount_one/_other) and the "1M+" overflow interpolation are actually exercised —
+// those are the two branches worth guarding. English is forced so the singular/plural strings are stable.
+import { describe, it, expect, beforeAll, afterEach } from "vitest";
+import { render, screen, cleanup } from "@testing-library/react";
+import "../../i18n"; // side-effect: initialise the shared i18next instance
+import i18n from "i18next";
+import ItemCount from "./ItemCount";
+
+beforeAll(async () => {
+  await i18n.changeLanguage("en");
+});
+afterEach(cleanup);
+
+const text = () => screen.getByTestId("item-count").textContent;
+
+describe("ItemCount", () => {
+  it("uses the singular string at 1 and the plural above it", () => {
+    render(<ItemCount count={1} />);
+    expect(text()).toBe("1 item");
+  });
+
+  it("pluralises and thousands-separates a normal count", () => {
+    render(<ItemCount count={192045} />);
+    // Separators come from toLocaleString (runtime-locale dependent) — compare against it, not a fixed
+    // en-US string, so the test asserts ItemCount's formatting path rather than the CI locale.
+    expect(text()).toBe(`${(192045).toLocaleString()} items`);
+  });
+
+  it("collapses a count of a million or more to 1M+, with the exact figure in the tooltip", () => {
+    render(<ItemCount count={1_000_000} />);
+    expect(text()).toBe("1M+ items"); // the "1M+" collapse is locale-independent
+    expect(screen.getByTestId("item-count").getAttribute("title")).toBe(
+      (1_000_000).toLocaleString(),
+    );
+  });
+
+  it("shows the icon-only fixed box (empty label) while the count is undefined", () => {
+    render(<ItemCount />);
+    expect(text()).toBe(""); // the Layers icon carries no text
+    expect(screen.getByTestId("item-count").getAttribute("title")).toBeNull();
+  });
+});