fix(browser): make startup reattach opt-in

[bobleer]

Follow-up to #2196, reported from use.

The problem

#2196 reattaches to a running browser on every BitFun start. That is free while the browser stays up, but the browser only keeps its approval grant for its own process lifetime. Whenever the browser had restarted since the last connection, the startup reattach raised an "Allow remote debugging?" dialog before the user had asked for the browser at all — which is what happened in practice.

Change

Gate it behind ai.browser_control_auto_connect_on_startup, off by default, as a switch under the Default CDP row it applies to.

The description states what the user is opting into rather than just naming the option:

BitFun 启动时，若浏览器正在运行且已启用上面的设置，就直接连上。注意：浏览器重启后会忘记之前的授权，这时会立刻弹出一次授权确认，即使你这次并不打算用浏览器。关闭时改为按需连接 —— Agent 第一次用到浏览器才连。

Attach as soon as BitFun starts, if the browser is already running with the setting above enabled. Note: the browser forgets its approval when it restarts, so after that you get an approval prompt right away — even if you weren't going to use the browser this session. When off, BitFun attaches the first time an agent needs the browser.

Off is not a regression from before #2196: the on-demand path still attaches the first time an agent needs the browser, and the "Ready, connects on use" status from #2196 already says so.

en-US / zh-CN / zh-TW all carry the full text.

Verified

cargo check -p bitfun-core -p bitfun-desktop, tsc --noEmit (0 errors), locale JSON validated, default confirmed false in both the struct and Default.