Chat mode, phase 1: grounded streaming chat over your notes

Entry points (no sixth tab): Ask in the Library header (desktop button,
phone icon), Ask about this deck in the deck menu, Ask a question on
study/homework results (compact result travels as context), and a Chat
row in the desktop sidebar. Phones get a full-screen route with back +
New chat; the tab bar steps aside.

- Client-side retrieval: chunk scoped decks (~800 tokens), BM25 against
  the question + recent turns, top chunks under ~7k tokens, each
  labelled with deck + section. Notes come from the deck's stored
  sourceText (already kept for Regenerate); decks without it fall back
  to generated content and the UI says so.
- Stateless server: /api/chat validates against the CHAT_MODELS
  allowlist, trims the history to ~6k tokens, caps context at ~9k and
  streams token/thinking/done/error NDJSON. /api/chat/extract reduces
  attachments to text through the existing vision step.
- UI: user bubbles + full-width assistant text, streaming with stop,
  auto-scroll that pauses with a jump-to-bottom button, safe markdown
  (React nodes, no HTML injection; KaTeX dynamically imported only when
  a message has math), Thinking row for reasoning, copy/regenerate/
  edit-and-resend, model + context pickers (last model remembered),
  composer with visualViewport keyboard handling, safe-area, 16px
  textarea, Enter-to-send on desktop.
- Usage-limit failures keep the draft and show the shared message with
  Settings/Ollama actions; stopping maps the reader abort to a clean
  stopped state; offline disables sending with a reason.
- Settings: Keep notes text for chat toggle; deck menu: Delete notes
  text. RATE_LIMIT_CHAT + CHAT_MODELS in .env.example and README.
- Tests: chat suite (63 checks, stub Ollama) wired into run-all.
  Full set: 12/12 suites, 487 checks. SW -> freezmo-v32.

Also: tests/model-compare.mjs (diagnostic) benchmarks two models on the
same notes (latency, repair rounds, activity types) so the gpt-oss vs
gemma default can be decided on numbers; defaults unchanged.

app/chat/page.tsx

@@ -0,0 +1,32 @@
+import { Suspense } from "react";
+
+import { ApiKeyGate } from "@/components/ApiKeyGate";
+import { AppHeader } from "@/components/AppHeader";
+import { Sidebar, SidebarOffset } from "@/components/Sidebar";
+import { ChatScreen } from "@/components/chat/ChatScreen";
+
+export const metadata = {
+  title: "Chat",
+};
+
+/**
+ * Chat mode. Phones get a full-screen conversation (the tab bar steps aside,
+ * see TabBar); desktop keeps the sidebar. The hidden AppHeader is only there
+ * to mount the settings dialog the sidebar opens.
+ */
+export default function ChatPage() {
+  return (
+    <ApiKeyGate>
+      <div className="flex min-h-dvh">
+        <Sidebar />
+        <SidebarOffset />
+        <main className="flex h-dvh min-h-dvh min-w-0 flex-1 flex-col">
+          <AppHeader className="hidden" />
+          <Suspense fallback={null}>
+            <ChatScreen />
+          </Suspense>
+        </main>
+      </div>
+    </ApiKeyGate>
+  );
+}