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/ChannelPage.tsx

@@ -5,7 +5,7 @@ import { useLiveQuery } from "../lib/useLiveQuery";
 import { channelDetailPollMs } from "../lib/channelDetailPoll";
 import { qk } from "../lib/queryKeys";
 import { useNavigationActions } from "./NavigationProvider";
-import { PriorityStepper } from "./PriorityStepper";
+import { PriorityStepper } from "./ui/PriorityStepper";
 import { useChannelPriority } from "../lib/useChannelPriority";
 import {
   ArrowLeft,
@@ -20,11 +20,11 @@ import {
   RefreshCw,
   SlidersHorizontal,
 } from "lucide-react";
-import Avatar from "./Avatar";
-import Tabs from "./Tabs";
+import Avatar from "./ui/Avatar";
+import Tabs from "./ui/Tabs";
 import Feed from "./Feed";
 import ChannelAboutContent from "./ChannelAbout";
-import { PageToolbar, PageToolbarSlot } from "./PageShell";
+import { PageToolbar, PageToolbarSlot } from "./ui/PageShell";
 import { useFeedFilters } from "./FeedFiltersProvider";
 import { useConfirm } from "./ConfirmProvider";
 import { notify } from "../lib/notifications";