Mobile message compose + WidgetConfigDialog SheetForms (Slice 8)

Below md, both the message-compose Dialog and the WidgetConfigDialog
render inside a SheetForm instead of a centered Dialog.

Message compose (UsersPage.impl.tsx): the form body (subject, formatting
toolbar, HTML textarea, preview, attachments) is extracted into a shared
composeBody const consumed by both SheetForm (mobile) and Dialog
(desktop). SheetForm wired with title, onSave=handleSend (which already
closes on success per R4.5), onCancel=closeCompose, isPending,
saveDisabled, saveLabel='Send message'.

WidgetConfigDialog: the draftBody const is shared between branches. The
two-mode flow (list vs draft) maps to dynamic SheetForm props -- list
mode ('Dashboard widgets' / Done / Cancel both close), draft mode
('Add/Edit widget' / Save widget / Cancel=reset back to list). The
inline Back/Save buttons are hidden on mobile (!isMobile) since the
SheetForm footer provides them.

Desktop (md+) is token-identical for both components -- the
isComposeMobile (900px) fullscreen styling on compose is preserved for
the 768-900px band. The large diff (~860 lines) is dominated by
extraction/re-indentation of shared form bodies into consts; the
behavioral delta is ~80 lines.

Tests: 3 new (compose mobile send/subject, WidgetConfigDialog desktop +
mobile titles/Done). 116 tests pass; lint/build green.

Refs openspec/changes/mobile-responsive-parity/ (spec R4, tasks slice 8).

frontend/src/components/__tests__/WidgetConfigDialog.test.tsx

@@ -0,0 +1,53 @@
+import { describe, it, expect, beforeEach, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { WidgetConfigDialog } from "../WidgetConfigDialog";
+
+// jsdom has no window.matchMedia; default to desktop (matches: false).
+function setMatchMedia(matches: boolean) {
+	window.matchMedia = ((query: string) => ({
+		matches: query.includes("768") ? matches : false,
+		media: query,
+		onchange: null,
+		addEventListener: () => {},
+		removeEventListener: () => {},
+		addListener: () => {},
+		removeListener: () => {},
+		dispatchEvent: () => false,
+	})) as unknown as typeof window.matchMedia;
+}
+
+vi.mock("../../hooks/useWidgets", () => ({
+	useWidgetInstances: () => ({ data: [] }),
+	useSaveWidgetInstance: () => ({ mutateAsync: vi.fn(), isPending: false }),
+	useDeleteWidgetInstance: () => ({ mutateAsync: vi.fn(), isPending: false }),
+}));
+
+vi.mock("../../hooks/useServices", () => ({
+	useServiceInstances: () => ({ data: [] }),
+}));
+
+vi.mock("../../hooks/useSettings", () => ({
+	useTasks: () => ({ data: [] }),
+}));
+
+beforeEach(() => setMatchMedia(false));
+
+describe("WidgetConfigDialog (desktop)", () => {
+	it("renders a Dialog with the dashboard widgets title at md+", () => {
+		render(<WidgetConfigDialog open={true} onClose={() => {}} />);
+		expect(
+			screen.getByRole("heading", { name: "Dashboard widgets" }),
+		).toBeInTheDocument();
+	});
+});
+
+describe("WidgetConfigDialog (mobile SheetForm — slice 8)", () => {
+	beforeEach(() => setMatchMedia(true));
+
+	it("renders a SheetForm with the dashboard widgets title below md", () => {
+		render(<WidgetConfigDialog open={true} onClose={() => {}} />);
+		expect(screen.getByText("Dashboard widgets")).toBeInTheDocument();
+		// List mode footer: "Done" button closes.
+		expect(screen.getByRole("button", { name: "Done" })).toBeInTheDocument();
+	});
+});