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/LibraryScreen.tsx

@@ -7,7 +7,7 @@ import { useEffect, useMemo, useRef, useState, useSyncExternalStore } from "reac
 import { DeckCard } from "@/components/DeckCard";
 import { FolderBar } from "@/components/FolderBar";
 import { HomeworkCard } from "@/components/HomeworkCard";
-import { EllipsisIcon, FolderIcon, UploadIcon } from "@/components/icons";
+import { ChatIcon, EllipsisIcon, FolderIcon, UploadIcon } from "@/components/icons";
 import { LinkLoadingScreen } from "@/components/LinkLoadingScreen";
 import { SegmentedPill } from "@/components/SegmentedPill";
 import { Button, EmptyState, ProgressBar } from "@/components/ui";
@@ -195,6 +195,10 @@ export function LibraryScreen({ initialTab = "decks" }: { initialTab?: "decks" |
         {/* Desktop keeps Import above the tabs; phones get a small icon
             button in the tab row instead. */}
         <div className="hidden flex-wrap items-center gap-2 sm:flex">
+          <Link href="/chat" className="touch-target inline-flex items-center gap-2 rounded-2xl border border-[var(--glass-border)] bg-[var(--surface-strong)] px-4 py-2.5 text-sm font-semibold text-[var(--text-muted)] transition hover:bg-[var(--hover)] hover:text-[var(--text)]">
+            <ChatIcon className="size-4" />
+            {t("chat.ask")}
+          </Link>
           <Button
             variant="secondary"
             size="md"
@@ -236,6 +240,14 @@ export function LibraryScreen({ initialTab = "decks" }: { initialTab?: "decks" |
               <span className="seg-label relative z-10">{t("tabHomework")}</span>
             </Link>
           </div>
+          <Link
+            href="/chat"
+            aria-label={t("chat.ask")}
+            title={t("chat.ask")}
+            className="touch-target grid size-11 shrink-0 place-items-center rounded-2xl border border-[var(--glass-border)] bg-[var(--surface-strong)] text-[var(--text-muted)] transition hover:bg-[var(--hover)] hover:text-[var(--text)] sm:hidden"
+          >
+            <ChatIcon className="size-5" />
+          </Link>
           <button
             type="button"
             onClick={() => setSheetOpen(true)}