feat(chat): unify session gateway with stable IDs and a single WS protocol

The frontend previously juggled placeholder IDs, provider-native IDs, and session_created handoffs, which caused race conditions and provider-specific branching. This introduces app-allocated session IDs, a chat run registry with event replay, delta sidebar updates, and one kind-based websocket contract so the UI can treat every provider the same while JSONL remains the source of truth.

server/shared/tests/slice-tail-page.test.ts

@@ -0,0 +1,42 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { sliceTailPage } from '@/shared/utils.js';
+
+const ITEMS = ['a', 'b', 'c', 'd', 'e'];
+
+test('offset 0 returns the most recent page', () => {
+  const { page, hasMore } = sliceTailPage(ITEMS, 2, 0);
+  assert.deepEqual(page, ['d', 'e']);
+  assert.equal(hasMore, true);
+});
+
+test('increasing offsets walk backwards in time', () => {
+  const { page, hasMore } = sliceTailPage(ITEMS, 2, 2);
+  assert.deepEqual(page, ['b', 'c']);
+  assert.equal(hasMore, true);
+});
+
+test('the oldest page reports hasMore false', () => {
+  const { page, hasMore } = sliceTailPage(ITEMS, 2, 4);
+  assert.deepEqual(page, ['a']);
+  assert.equal(hasMore, false);
+});
+
+test('null limit returns everything', () => {
+  const { page, hasMore } = sliceTailPage(ITEMS, null, 0);
+  assert.deepEqual(page, ITEMS);
+  assert.equal(hasMore, false);
+});
+
+test('offsets past the start return an empty page', () => {
+  const { page, hasMore } = sliceTailPage(ITEMS, 3, 10);
+  assert.deepEqual(page, []);
+  assert.equal(hasMore, false);
+});
+
+test('zero limit returns an empty page but keeps hasMore accurate', () => {
+  const { page, hasMore } = sliceTailPage(ITEMS, 0, 0);
+  assert.deepEqual(page, []);
+  assert.equal(hasMore, true);
+});