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/components/copilot.js

@@ -162,11 +162,8 @@ export class AppCopilot extends I18nMixin(ChatSession) {
     return this.querySelector('.copilot-textarea');
   }
 
-  _scrollToBottom() {
-    this.updateComplete.then(() => {
-      const el = this.querySelector('.copilot-messages');
-      if (el) el.scrollTop = el.scrollHeight;
-    });
+  _messagesContainer() {
+    return this.querySelector('.copilot-messages');
   }
 
   _onMessagePushed(item) {
@@ -389,6 +386,15 @@ export class AppCopilot extends I18nMixin(ChatSession) {
             ${t('chat.thinking')}
           </div>
         ` : nothing}
+
+        ${this._showJump ? html`
+          <button class="copilot-jump-btn" type="button"
+                  title=${t('chat.scroll_to_latest')}
+                  aria-label=${t('chat.scroll_to_latest')}
+                  @click=${() => this._jumpToBottom()}>
+            <i class="bi bi-arrow-down-circle-fill"></i>
+          </button>
+        ` : nothing}
       </div>
 
       <div class="copilot-input-area">

web/components/shared/chat-page.js

@@ -43,7 +43,7 @@ export class ChatPage extends ChatSession {
 
   updated(changed) {
     if (changed.has('visible') && this.visible) {
-      this._scrollToBottom();
+      this._forceScrollToBottom();
     }
     // The owner (mobile-app) re-points this chat by changing `source`. Switch the
     // live connection — base `_switchSource` tears down the WS, reloads that
@@ -72,11 +72,8 @@ export class ChatPage extends ChatSession {
     return this.querySelector('.chat-page-textarea');
   }
 
-  _scrollToBottom() {
-    this.updateComplete.then(() => {
-      const el = this.querySelector('.chat-page-messages');
-      if (el) el.scrollTop = el.scrollHeight;
-    });
+  _messagesContainer() {
+    return this.querySelector('.chat-page-messages');
   }
 
   _onMessagePushed(item) {
@@ -184,6 +181,15 @@ export class ChatPage extends ChatSession {
               ${t('chat.thinking')}
             </div>
           ` : nothing}
+
+          ${this._showJump ? html`
+            <button class="copilot-jump-btn" type="button"
+                    title=${t('chat.scroll_to_latest')}
+                    aria-label=${t('chat.scroll_to_latest')}
+                    @click=${() => this._jumpToBottom()}>
+              <i class="bi bi-arrow-down-circle-fill"></i>
+            </button>
+          ` : nothing}
         </div>
 
         <div class="chat-page-input-area">