[Feature]: Setting to hide the thread minimap

[HarjjotSinghh]

Area

apps/web

Problem or use case

The thread minimap (the tick-mark strip on the left of the chat, added in #1915) is always on, with no way to turn it off.

I never use it to navigate, but it permanently takes up the left gutter, adds visual noise next to every message, and pops hover previews over the chat content whenever my cursor passes near that edge.

For people who navigate long threads by scrolling or search, it's pure clutter with no off switch.

Proposed solution

Add a "Show thread minimap" toggle under Settings > Behavior, default ON so nothing changes for existing users.

When OFF:

• Don't render the minimap or its hover previews
• Remove its layout gutter so the chat column reclaims the space
• Hide the previous/next turn arrows from feat(web): add previous/next turn navigation in minimap #8531 too, since they live on the minimap

Smallest useful scope

A single persisted boolean in behavior settings that skips rendering the minimap component. Gutter removal and a keybinding to toggle it can follow.