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/Playlists.tsx

@@ -42,8 +42,8 @@ import { cycleSort, type SortState } from "../lib/columnSort";
 import { formatRuntime } from "../lib/format";
 import { applyView, sameOrder } from "../lib/playlistView";
 const PlayerModal = lazy(() => import("./PlayerModal"));
-import UndoToolbar from "./UndoToolbar";
-import { PageToolbar } from "./PageShell";
+import UndoToolbar from "./ui/UndoToolbar";
+import { PageToolbar } from "./ui/PageShell";
 import { useChoice, useConfirm } from "./ConfirmProvider";
 import { usePlaylists, usePlaylistsActions } from "./PlaylistsProvider";
 import { useMe } from "../lib/useMe";
@@ -54,7 +54,7 @@ import {
   isActive,
   type Column,
   type FilterMap,
-} from "./tableHeader";
+} from "./ui/tableHeader";
 import { GRIP_KEY, playlistColumns } from "./playlistColumns";
 
 // The playlist detail is a HYBRID table (E4 S4): real sortable/filterable column headers over a