Reader: stale reading sessions never move "Continue" back

A reader tab left open on one device (or restored by the browser) used to
save its old chapter as the newest position as soon as it was shown again.
Progress saves now carry the tab id and the newest progress time the tab
knows (`seen`); the server refuses a save when another tab or device saved
a newer position since. The reader then asks: jump to the newest position,
or keep reading here (which makes this chapter the head again). It checks on
open, when the tab becomes visible, and on every save. Reader links carry
`at` and the reader keeps it in the address bar, so reloads and restored
tabs still know what they had seen.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/app.css

@@ -429,6 +429,14 @@ body.reader { position: fixed; top: 0; left: 0; right: 0; bottom: 0; overflow: h
 .reading-gate-message[data-tone="retry"] { border-color: #9b7233; background: #382b1a; color: #ffdaa0; }
 .reading-gate-message[data-tone="error"] { border-color: #944c55; background: #3c2025; color: #ffc0c6; }
 @media (max-width: 660px) { .reading-gate { margin: 28px 12px 100px; padding: 18px; } }
+/* Stale reading session: a newer position was saved elsewhere since this page was opened. */
+.stale-session { position: fixed; inset: 0; z-index: 60; display: flex; align-items: center; justify-content: center;
+  padding: 16px; background: rgba(0, 0, 0, .6); }
+.stale-box { box-sizing: border-box; width: min(440px, 100%); padding: 20px; border-radius: 12px; background: var(--card);
+  border: 1px solid var(--line); box-shadow: 0 12px 40px rgba(0, 0, 0, .6); }
+.stale-box h2 { margin: 0 0 8px; font-size: 18px; }
+.stale-actions { display: flex; flex-wrap: wrap; gap: 10px; margin: 16px 0 10px; }
+.stale-actions .btn { flex: 1 1 auto; justify-content: center; text-align: center; }
 .pg { position: absolute; overflow: hidden; background: #19191c; }
 .pg img { display: block; width: 100%; height: 100%; -webkit-user-drag: none; }
 #canvas[data-page-tone="inverted"] .pg img { filter: invert(1); }