Fix wave chunk B: shared MenuShell for model menus, setup CTA on /chat/new

- components/menu-shell.tsx: one portaled, fixed-anchored, viewport-capped
  (60vh pattern) menu shell with site-preferred side + honest-fit auto-flip;
  computeMenuPlacement exported pure, placement-only (sites keep their own
  open/close/Esc contracts); re-places on capture-phase scroll/resize
- migrated: settings slot menus (agent-section), keys Add-provider,
  composer model+thinking (open up, right-aligned), SetupDialog pickers
  (portaled out of the overflow-hidden dialog frame — no clipping by
  construction); graph popovers + stats-bar summary stay bespoke (owner)
- /chat/new: the setup CTA above the draft blob, mirroring home's
- tests: menu-shell-pure (geometry/cap/flip/alignment), DraftShell CTA

components/model-picker.tsx

@@ -6,6 +6,7 @@ import type { ThinkingLevel } from "@earendil-works/pi-agent-core";
 import { prettyModel } from "@/components/usage-format";
 import { ModelMenuList, ThinkingMenuList } from "@/components/settings/model-menu";
 import { composeSlotModel } from "@/components/settings/agent-section";
+import { MenuShell } from "@/components/menu-shell";
 
 /** GET /api/models is cached per page load for instant menu opens, but
  *  the federation is LIVE per call — keys saved mid-session widen the
@@ -252,9 +253,11 @@ export function ModelControl() {
 			    settings' slot selects and the setup dialog use the exact same
 			    one. */}
 			{open === "model" && (
-				<div
-					data-testid="model-menu"
-					className="absolute bottom-full right-0 z-40 mb-1.5 flex w-[22rem] flex-col rounded-xl border bg-card p-2 text-xs shadow-xl"
+				<MenuShell
+					anchorRef={containerRef}
+					testid="model-menu"
+					preferredSide="up"
+					menuClass="flex w-[22rem] flex-col rounded-xl border bg-card p-2 text-xs shadow-xl"
 				>
 					{error && <div className="mb-1.5 text-destructive">{error}</div>}
 					<ModelMenuList
@@ -268,13 +271,15 @@ export function ModelControl() {
 								: "Strong tier — runs prober + teacher, app-wide"
 						}
 					/>
-				</div>
+				</MenuShell>
 			)}
 
 			{open === "thinking" && (
-				<div
-					data-testid="thinking-menu"
-					className="absolute bottom-full right-0 z-40 mb-1.5 flex w-44 flex-col gap-0.5 rounded-xl border bg-card p-2 text-xs shadow-xl"
+				<MenuShell
+					anchorRef={containerRef}
+					testid="thinking-menu"
+					preferredSide="up"
+					menuClass="flex w-44 flex-col gap-0.5 rounded-xl border bg-card p-2 text-xs shadow-xl"
 				>
 					<ThinkingMenuList
 						levels={catalog ? levels : []}
@@ -286,7 +291,7 @@ export function ModelControl() {
 						footer="Levels this model actually supports"
 					/>
 					{error && <div className="mt-1 text-destructive">{error}</div>}
-				</div>
+				</MenuShell>
 			)}
 		</div>
 	);