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/PersonalViewsModal.tsx

@@ -17,13 +17,13 @@ import {
 } from "@dnd-kit/sortable";
 import { CSS } from "@dnd-kit/utilities";
 import { coercePlexFilters, type FeedFilters, type PlexFilters, type SavedView } from "../lib/api";
-import Modal from "./Modal";
+import Modal from "./ui/Modal";
 import { Button } from "./ui/Button";
 import { useSavedViews, asFilters } from "./SavedViewsProvider";
 import { useActiveFilters } from "../lib/useActiveFilters";
 import { buildPlexActiveFilters } from "./usePlexActiveFilters";
 import { useUndoable } from "../lib/useUndoable";
-import UndoToolbar from "./UndoToolbar";
+import UndoToolbar from "./ui/UndoToolbar";
 
 // Master–detail manager for saved ("personal") views: the left column lists the views (drag to
 // reorder) plus a "save the current filters" input; the right column shows the selected view's name