[Desktop/Web] Model selector should scale to the available screen size (fixed ~284px popover feels cramped)

Description

When I open the model selector ("Choose model") in OpenCode Desktop, it renders as a very small, compressed popover. I'm on a large monitor with plenty of free space, yet the list only shows a handful of rows before scrolling and long model names get truncated. It feels cramped for no reason.

Current behavior

• The popover has a fixed, narrow width. In packages/app/src/components/dialog-select-model.tsx the content is rendered with class="w-[284px] overflow-hidden ...".
• The list keeps a short visible height (with a sticky provider group label), so only a few models are visible at a time.
• Long model names (e.g. claude-sonnet-4-..., gpt-...) are truncated even inside the dropdown, where there is clearly room.

Expected behavior

• The selector should scale with the window/viewport: a taller list (more visible rows) and a wider popover when the screen allows it, instead of a fixed tiny box.
• Keep a sensible upper bound (e.g. max-h/max-w relative to viewport) and stay scrollable on small screens.
• Show full model names in the list (no ellipsis) when there is available space.
• Preserve the current compact behavior on small windows/mobile.

Why this matters

• Users with large screens waste space and scroll unnecessarily to find a model.
• The number of models is large (OpenCode Zen alone lists ~79), so seeing only a handful of rows makes browsing painful.

Related

• #23054 (closed as not planned) - "Can this popup be made larger? The name is being cut off."
• #20968 - web: agent/model dropdown truncates full model names instead of showing complete text
• #26989 (closed as not planned) - The main interface is ugly with a long model name

Environment

• UI: OpenCode Desktop (also affects Web)
• OS: Windows