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/lib/channelsUrlState.ts

@@ -6,7 +6,7 @@
 // emitted, so URLs stay clean. The manager's name search is deliberately EPHEMERAL (like the feed/plex
 // rail search) and stays OUT of the shared contract.
 import { decodeSortParam, encodeSortParam, type SortState } from "./columnSort";
-import { type FilterMap } from "../components/tableHeader";
+import { type FilterMap } from "../components/ui/tableHeader";
 
 // The content state S8 mirrors. `view` is the raw tab string (the provider clamps it); `sort` is the
 // raw provider SortState (healed against live columns only at the point of use), so the user's real