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/eslint.config.js

@@ -67,5 +67,34 @@ export default tseslint.config(
       ],
     },
   },
+  {
+    // E10 S1: `components/ui/` is the PLATFORM layer, and the rule that makes it a layer rather than
+    // a folder is enforced here, not in prose (see components/ui/README.md). A platform component is
+    // domain-free: it may import sibling primitives and generic `lib/` helpers, never a feature
+    // component (`../Feed`) and never a domain API slice. Without this rule the boundary erodes on
+    // the first "just this once" import and the directory silently becomes another components/.
+    files: ["src/components/ui/**/*.{ts,tsx}"],
+    rules: {
+      "no-restricted-imports": [
+        "error",
+        {
+          patterns: [
+            {
+              // `../X` (exactly one level up = a feature component). A `regex` rather than a glob:
+              // eslint matches `patterns` gitignore-style, where `../*` also swallows `../../lib/x`.
+              regex: "^\.\./(?!\.\./)",
+              message:
+                "components/ui/ is domain-free: a platform component may not import a feature component. Invert the dependency (take a prop/render-prop) or keep the component outside ui/.",
+            },
+            {
+              regex: "^\.\./\.\./lib/api/",
+              message:
+                "components/ui/ is domain-free: a platform component may not call a domain API slice. Take the data as props.",
+            },
+          ],
+        },
+      ],
+    },
+  },
   prettier,
 );