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/homework/HomeworkGuide.tsx

@@ -7,6 +7,7 @@ import { ActivityRenderer } from "@/components/activities";
 import { PromptCard, QuestionText } from "@/components/activities/shared";
 import { XIcon } from "@/components/icons";
 import { Button, ProgressBar } from "@/components/ui";
+import { AskButton } from "@/components/chat/AskButton";
 import { assembleSolution } from "@/lib/homework";
 import { useI18n } from "@/lib/i18n";
 import type { HomeworkGuideRecord, HomeworkStep } from "@/lib/schemas";
@@ -283,6 +284,18 @@ export function HomeworkGuide({
             <p className="text-[12.5px] text-[var(--text-subtle)]">{t("homework.guide.solutionNote")}</p>
             <div className="flex flex-wrap items-start gap-2">
               <CopyButton text={solution} />
+              <AskButton
+                seed={{
+                  label: record.title,
+                  text: [
+                    `${t("chat.seedAssignment")}:`,
+                    record.sourceText,
+                    "",
+                    `${t("chat.seedSteps")}:`,
+                    ...steps.map((step, index) => `${index + 1}. ${step.title}${step.explanation ? ` — ${step.explanation}` : ""}`),
+                  ].join("\n"),
+                }}
+              />
               <Button variant="secondary" className="touch-target" onClick={onAgain} loading={againLoading}>
                 {t("homework.followup.redo")}
               </Button>