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/en.ts

@@ -240,6 +240,7 @@ export const enLocale: LocalePlugin = {
     "chat.commandCopy": "Copy the last assistant message",
     "chat.compacted": "Compacted",
     "chat.tokensSaved": "{saved} saved",
+    "chat.scrollToBottom": "Scroll to bottom",
     "mcp.addServer": "Add server",
     "mcp.addEnv": "Add env",
     "mcp.serverName": "Server name",