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/OnboardingWizard.tsx

@@ -5,7 +5,7 @@ import { qk } from "../lib/queryKeys";
 import { AlertTriangle, Check, Loader2, RefreshCw, ShieldCheck, X, Youtube } from "lucide-react";
 import { api, type Me } from "../lib/api";
 import { beginGrant, endOnboarding } from "../lib/onboarding";
-import Overlay from "./Overlay";
+import Overlay from "./ui/Overlay";
 
 // A first-login wizard that walks the user through granting YouTube access one scope at a
 // time, each with a plain explanation and an up-front heads-up about Google's "unverified