SheetForm dirty-state confirm + wire isDirty into all form consumers (R4.5)

SheetForm gains an isDirty prop. When true, any close attempt (Cancel
button, header X, Radix overlay click, Escape) opens a 'Discard changes?'
ConfirmDialog instead of discarding unsaved edits. Radix dismiss callbacks
(onEscapeKeyDown, onPointerDownOutside) are intercepted when dirty so the
guard applies uniformly.

All four form consumers now compute and pass isDirty:
- ServicePage: name/enabled/config differ from the persisted instance.
- Settings machine editor: field-by-field draft vs editingMachine
  (create mode is always dirty; secret write-only fields excluded).
- Message compose: subject non-empty, body differs from default, or
  attachments present.
- WidgetConfigDialog: draft !== null (only draft mode is guarded; list
  mode has nothing to discard).

Tests: 3 new SheetForm dirty-guard cases (prompt on cancel, abort discard,
clean close when not dirty) + one focused dirty-guard test per consumer.
122 tests pass; lint/build green.

Refs openspec/changes/mobile-responsive-parity/verify-report.md residual
risk #1.

frontend/src/pages/__tests__/Settings.test.tsx

@@ -187,4 +187,27 @@ describe("Settings (mobile SheetForm — slice 7)", () => {
 		// (The page content itself is still rendered; only the sheet unmounts.)
 		expect(screen.queryByText("Edit machine")).not.toBeInTheDocument();
 	});
+
+	it("prompts before discarding unsaved machine edits (R4.5)", async () => {
+		machines = [localMachine()];
+		render(<Settings />);
+
+		const detailEdit = screen
+			.getAllByRole("button", { name: "Edit" })
+			.find((button) => button.textContent === "Edit") as HTMLButtonElement;
+		await userEvent.click(detailEdit);
+
+		// Edit the name to make the form dirty.
+		const nameInput = screen.getByLabelText("Name");
+		await userEvent.clear(nameInput);
+		await userEvent.type(nameInput, "Dirty name");
+
+		// Cancel should NOT immediately close — the discard confirm appears.
+		await userEvent.click(screen.getByRole("button", { name: "Cancel" }));
+		expect(
+			screen.getByRole("heading", { name: "Discard changes?" }),
+		).toBeInTheDocument();
+		// The editor is still open.
+		expect(screen.getByText("Edit machine")).toBeInTheDocument();
+	});
 });