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/VideoInfoCard.tsx

@@ -6,8 +6,8 @@ import { qk } from "../lib/queryKeys";
 import { api, type Video } from "../lib/api";
 import { formatDate, formatDuration, formatViews } from "../lib/format";
 import { linkify } from "../lib/linkify";
-import Avatar from "./Avatar";
-import { PopoverSurface, useAnchoredPopover } from "./AnchoredPopover";
+import Avatar from "./ui/Avatar";
+import { PopoverSurface, useAnchoredPopover } from "./ui/AnchoredPopover";
 
 // The feed card's "more than the card face" hover infobox (E8·S3, decision D3): full (un-clamped)
 // title, channel, views·duration·date, a description snippet, and the channel's tags. It hangs off a