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.

components/Sidebar.tsx

@@ -4,6 +4,7 @@ import Link from "next/link";
 import { useMemo, useState, useSyncExternalStore, type ReactNode } from "react";
 
 import {
+  ChatIcon,
   ChevronLeftIcon,
   ChevronRightIcon,
   CompassIcon,
@@ -137,6 +138,16 @@ function NavContent({ collapsed = false }: { collapsed?: boolean }) {
         {!collapsed ? <span className="flex-1 truncate text-left">{t("homework.nav")}</span> : null}
       </Link>
 
+      {/* chat with your notes --------------------------------------------- */}
+      <Link
+        href="/chat"
+        className={cn("nav-row mb-3", collapsed && "justify-center px-2")}
+        title={t("chat.nav")}
+      >
+        <ChatIcon className="size-4 shrink-0" />
+        {!collapsed ? <span className="flex-1 truncate text-left">{t("chat.nav")}</span> : null}
+      </Link>
+
       <nav aria-label={t("sidebar.library")} className="thin-scroll min-h-0 flex-1 space-y-0.5 overflow-y-auto">
         {collapsed ? (
           <button