Fix dev bulk-delete button gate + reposition inside campaigns

Gate was process.env.NODE_ENV === 'development' — unsafe default. react-scripts
inlines NODE_ENV=development when unset, so prod deploys forgetting the env var
exposed the delete-all button. Switched to explicit opt-in REACT_APP_DEV_TOOLS=1.

process.env.REACT_APP_DEV_TOOLS as static literal gets inlined by DefinePlugin at
webpack build time — runtime mutations in tests had no effect, and dev-start
without the env produced bundles with the branch dead-stripped. Extracted gate
to src/config/devTools.js using dynamic key access
(process.env['REACT_APP_' + 'DEV_TOOLS']) so DefinePlugin cannot inline it; the
value is read at runtime. dev-start.sh now exports REACT_APP_DEV_TOOLS=1.

Button had also drifted outside the campaigns collapse block to the page bottom;
moved it back inside the campaigns section after the grid.

Tests cover both paths: gate logic (unset/0/arbitrary/1) in BulkDelete.gate.test.js,
prod safety render (button absent when unset) in BulkDelete.render-hidden.test.js,
dev feature render (button present when DEV_TOOLS=1) in BulkDelete.render-shown.test.js.

src/tests/BulkDelete.render-shown.test.js

@@ -0,0 +1,36 @@
+import React from 'react';
+import { render, screen, waitFor, fireEvent, cleanup, act } from '@testing-library/react';
+import App from '../App';
+
+describe('Bulk-delete-all button dev feature', () => {
+  const orig = process.env.REACT_APP_DEV_TOOLS;
+
+  afterEach(() => {
+    cleanup();
+    if (orig === undefined) delete process.env.REACT_APP_DEV_TOOLS;
+    else process.env.REACT_APP_DEV_TOOLS = orig;
+  });
+
+  async function renderAndCreateCampaign() {
+    window.history.replaceState({}, '', '/');
+    global.alert = jest.fn();
+    global.window.open = jest.fn();
+    render(<App />);
+    await waitFor(() => screen.getByRole('button', { name: /Create Campaign/i }));
+    await act(async () => {
+      fireEvent.click(screen.getByRole('button', { name: /Create Campaign/i }));
+    });
+    await waitFor(() => screen.getByLabelText(/Campaign Name/i));
+    await act(async () => {
+      fireEvent.change(screen.getByLabelText(/Campaign Name/i), { target: { value: 'Gate Render Test' } });
+      fireEvent.click(screen.getByRole('button', { name: /^Create$/i }));
+      await new Promise(r => setTimeout(r, 200));
+    });
+  }
+
+  test('dev: button present when DEV_TOOLS=1 and campaigns exist', async () => {
+    process.env.REACT_APP_DEV_TOOLS = '1';
+    await renderAndCreateCampaign();
+    await waitFor(() => screen.getByText(/Delete All Campaigns/i), { timeout: 5000 });
+  });
+});