Preserve preview harness, editor, email and task improvements

Snapshot current maintainer-preview application changes and regression fixtures for integration into lab. Excludes local runtime data, evaluation outputs and source backups. Focused Python regression selection: 140 passed; full suite not certified.

tests/sidebarNewChat.test.mjs

@@ -0,0 +1,39 @@
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import { runInNewContext } from 'node:vm';
+import test from 'node:test';
+
+const source = readFileSync(new URL('../static/app.js', import.meta.url), 'utf8');
+const start = source.indexOf("  const sidebarNewChatBtn = el('sidebar-new-chat-btn');");
+const end = source.indexOf('  // Delete session button on icon rail', start);
+assert.ok(start >= 0 && end > start);
+
+for (const width of [390, 767, 768, 1280]) {
+  test(`New Chat drawer behavior at ${width}px`, async () => {
+    let click;
+    let calls = 0;
+    let syncs = 0;
+    let finish;
+    const pending = new Promise(resolve => { finish = resolve; });
+    const sidebar = new Set();
+    const backdrop = new Set(['visible']);
+    const nodes = {
+      'sidebar-new-chat-btn': { addEventListener: (event, handler) => { click = handler; } },
+      sidebar: { classList: { add: value => sidebar.add(value) } },
+      'sidebar-backdrop': { classList: { remove: value => backdrop.delete(value) } },
+    };
+    runInNewContext(source.slice(start, end), {
+      el: id => nodes[id],
+      window: { innerWidth: width, syncRailSide: () => { syncs++; } },
+      _handleNewChatAction: () => { calls++; return pending; },
+    });
+    const result = click({ preventDefault() {}, stopImmediatePropagation() {} });
+    // Drawer closes before asynchronous model/session setup finishes.
+    assert.equal(sidebar.has('hidden'), width < 768);
+    assert.equal(backdrop.has('visible'), width >= 768);
+    assert.equal(syncs, width < 768 ? 1 : 0);
+    assert.equal(calls, 1);
+    finish();
+    await result;
+  });
+}