Browser pane: add a mute control (and default-mute) for agent-opened tabs

What's missing
The built-in browser pane (mcp__Claude_Browser__*) has no way to mute a tab. When research opens a page that autoplays audio — YouTube being the obvious case — the sound plays out loud.

Requested

1. A mute control on the pane's tabs, and a muted parameter on preview_start / navigate / tabs_create.
2. A user setting to default agent-opened tabs to muted, since agent research almost never needs audio.

Why
Users often have the agent researching in the background while doing something else. An unexpected autoplay burst is disruptive, and there is currently no reliable, site-agnostic way for the agent to prevent it.