Digest CTA: clear pending opt-in if sign-in is cancelled

Per Codex polish: if someone taps 'Get tomorrow's brief' then dismisses sign-in
without authing, clear pendingDigestOptIn (guarded by !auth.user so a successful
sign-in still auto-enables via the $effect).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -405,7 +405,7 @@
 
 <Header onSaved={() => (showSaved = true)} onaccount={openAccount} user={auth.user} boundariesActive={filtersOn} />
 
-{#if showSignIn}<SignIn onclose={() => (showSignIn = false)} />{/if}
+{#if showSignIn}<SignIn onclose={() => { showSignIn = false; if (!auth.user) pendingDigestOptIn = false; }} />{/if}
 {#if showSaved && auth.user}<SavedFlyout onclose={() => (showSaved = false)} />{/if}
 {#if showLanes && lanePool}
   <LanePicker pool={lanePool} selected={pinnedLaneKeys} onsave={saveLanes} onclose={() => (showLanes = false)} />