Mute preview tabs by default, plus "Mute all" / "Close all preview tabs"

[rene-demonsters]

Problem

Agents often open the app in the preview side panel to check their changes. For projects with background audio (games, media apps, anything that autoplays), every preview tab plays its own audio. The tabs pile up across threads, so the same sound ends up playing many times at once.

The lifecycle bug behind this is tracked in #15579: preview tabs are never closed or suspended. Even with that fixed, it can be hard to know when a preview is truly done with, and audio is the symptom that disrupts work right away. So it would help to have user-facing controls that don't depend on getting the lifecycle right.

Proposal

• Setting: "Mute preview tabs by default" (off by default). Every preview tab starts muted, and you unmute a tab from the panel when you want to hear it.
• Auto-mute hidden previews: mute any preview tab that isn't the visible one in the active thread.
• Audio indicator: show a speaker icon on preview tabs that are playing sound, like browsers do, so you can see where the noise comes from.
• "Mute all" / "Close all preview tabs" actions, across all threads, in the command palette or the preview panel menu.

Why this is cheap

Electron already supports per-webview muting (webContents.setAudioMuted() and isCurrentlyAudible()), so the setting and the indicator shouldn't need much new plumbing.

Related: #15579, #20890