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.gate.test.js

@@ -0,0 +1,30 @@
+import { isDevToolsEnabled } from '../config/devTools';
+
+describe('Bulk-delete-all dev-tools gate logic', () => {
+  const orig = process.env.REACT_APP_DEV_TOOLS;
+
+  afterEach(() => {
+    if (orig === undefined) delete process.env.REACT_APP_DEV_TOOLS;
+    else process.env.REACT_APP_DEV_TOOLS = orig;
+  });
+
+  test('false when unset (safe default)', () => {
+    delete process.env.REACT_APP_DEV_TOOLS;
+    expect(isDevToolsEnabled()).toBe(false);
+  });
+
+  test('false when 0', () => {
+    process.env.REACT_APP_DEV_TOOLS = '0';
+    expect(isDevToolsEnabled()).toBe(false);
+  });
+
+  test('false when arbitrary string', () => {
+    process.env.REACT_APP_DEV_TOOLS = 'true';
+    expect(isDevToolsEnabled()).toBe(false);
+  });
+
+  test('true only when exactly 1', () => {
+    process.env.REACT_APP_DEV_TOOLS = '1';
+    expect(isDevToolsEnabled()).toBe(true);
+  });
+});