chat: stick-to-bottom auto-scroll with jump-to-latest button

Auto-scroll now yields when the reader scrolls away from the bottom, so a
fast-streaming reply no longer fights someone reading the start. Stickiness is
a flag driven by a passive scroll listener (not a per-flush distance check,
which breaks when one flush adds more than the threshold of content); scrolling
back within the band re-arms it.

Centralised in chat-session.js (_scrollToBottom(force) + _forceScrollToBottom
+ _messagesContainer hook), removing the duplicated overrides in copilot.js /
chat-page.js. Force-scroll is used where the latest must be shown: history
load, chat reopen, approval/clarification prompts, and message send.

A sticky 'jump to latest' affordance appears only while scrolled up.

web/css/copilot-messages.css

@@ -9,6 +9,39 @@
   gap: 0.7rem;
 }
 
+/* ── "Jump to latest" floating affordance ──────────────────────────────────── */
+/* Shown only while the reader is scrolled away from the bottom (auto-scroll has
+   yielded). Sticky to the scrollport bottom so it stays in view as they read;
+   removed from the DOM entirely while stuck to the bottom. */
+.copilot-jump-btn {
+  position: sticky;
+  bottom: 0.6rem;
+  align-self: flex-end;
+  width: 38px;
+  height: 38px;
+  display: grid;
+  place-items: center;
+  padding: 0;
+  border: 1px solid var(--card-border);
+  border-radius: 50%;
+  background: rgba(var(--accent-rgb), 0.92);
+  color: #fff;
+  box-shadow: var(--card-shadow);
+  cursor: pointer;
+  z-index: 5;
+  font-size: 1.1rem;
+  line-height: 1;
+  transition: transform 0.12s ease, background-color 0.12s ease;
+}
+.copilot-jump-btn:hover { background: var(--accent); transform: translateY(-1px); }
+.copilot-jump-btn:active { transform: translateY(0); }
+.copilot-jump-btn:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }
+
+@media (prefers-reduced-motion: reduce) {
+  .copilot-jump-btn { transition: none; }
+  .copilot-jump-btn:hover { transform: none; }
+}
+
 /* ── Message bubbles ───────────────────────────────────────────────────────── */
 
 .copilot-msg {