fix(dashboard): lead with the stored lines for a digest cached before the lead

The hero reads only digest.lead. A digest the service worker cached
before the update carries no such field, so the hero lost its lead line
until the next fetch. Such a digest now falls back to the reaction line
or the briefing lead it was built to show; a current digest whose lead
is null stays silent.

src/components/daily/__tests__/today-hero.test.tsx

@@ -185,6 +185,37 @@ describe("<TodayHero>", () => {
     expect(html).toContain('data-slot="today-hero-lead"');
   });
 
+  it("falls back to the stored lines for a digest cached before the lead existed", () => {
+    // The service worker's offline cache can hand back a digest written by
+    // the previous version, which carries no `lead` field at all.
+    const cached = digest({ reactionLine: "The new reading fits your week." });
+    delete (cached as Partial<DailyDigest>).lead;
+    const html = render(<TodayHero digest={cached} />);
+    expect(visibleText(html)).toContain("The new reading fits your week.");
+
+    const briefingOnly = digest();
+    delete (briefingOnly as Partial<DailyDigest>).lead;
+    expect(visibleText(render(<TodayHero digest={briefingOnly} />))).toContain(
+      "Your week is trending steady.",
+    );
+  });
+
+  it("does not resurrect a line the server left out of the lead", () => {
+    // A current digest with `lead: null` decided there is nothing to say;
+    // its reaction line (e.g. one that only repeated the score) stays out.
+    const html = render(
+      <TodayHero
+        digest={digest({
+          lead: null,
+          briefingLead: null,
+          topSignal: null,
+          reactionLine: "Score 82 today.",
+        })}
+      />,
+    );
+    expect(visibleText(html)).not.toContain("Score 82 today.");
+  });
+
   it("does not repeat the top signal under a lead made from it", () => {
     // A briefing whose only sentence repeated the score leads with the top
     // signal's headline (resolved on the server); the muted signal line under

src/components/daily/today-hero.tsx

@@ -141,8 +141,13 @@ export function TodayHero({
   // a briefing sentence with content while AI text is available, otherwise a
   // deterministic sentence about the day's strongest signal. Greetings and a
   // sentence that only repeats the ring's number never reach it, so the hero
-  // renders it verbatim and decides nothing about it.
-  const lead = digest.lead?.text ?? null;
+  // renders it verbatim and decides nothing about it. A digest the service
+  // worker cached before the field existed has no `lead` at all; it falls
+  // back to the lines that version led with.
+  const lead =
+    digest.lead === undefined
+      ? (digest.reactionLine ?? digest.briefingLead ?? null)
+      : (digest.lead?.text ?? null);
   const topSignal = digest.topSignal;
   // The briefing's top signal rides under an AI lead as its supporting line,
   // and only with what the lead does not already say: a headline the lead