feat(web): auto-follow streaming output and add scroll-to-bottom button

- ChatWindow: follow the streaming bubble on every chunk (scrollIntoView block:end),
  driven by followStreamRef (true by default, only genuine scroll gestures flip it)
- Remove the agentRunning spacer that pushed content above the viewport at the bottom
- Move the scroll-to-bottom button into the input row (ChatInput scrollToBottomButton slot),
  always visible, dimmed at the bottom; click jumps to the live stream or the end
- useAgentSession: drop window pointerdown as scroll-intent marker (send-click was
  misread as a scroll-away, disabling follow before the stream started); expose
  atBottom/followStreamRef/scrollToBottom
- i18n: add chat.scrollToBottom (en/zh-CN)

pi-web/lib/i18n/messages/zh-CN.ts

@@ -240,6 +240,7 @@ export const zhCNLocale: LocalePlugin = {
     "chat.commandCopy": "复制最后一条助手消息",
     "chat.compacted": "已压缩",
     "chat.tokensSaved": "节省 {saved}",
+    "chat.scrollToBottom": "滚动到底部",
     "mcp.addServer": "添加服务器",
     "mcp.addEnv": "添加环境变量",
     "mcp.serverName": "服务器名称",