## Problem
The model selection dropdown doesn't scroll when there are many models
available, causing the list to extend beyond the viewport and become
unusable.

## Solution
- Added `max-h-100 overflow-y-auto` classes to the DropdownMenuContent

This closes (#1279)

    
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## Summary by cubic
Adds vertical scrolling to the model selection dropdown so long lists
don’t overflow the viewport. Applies max height and overflow-y auto to
DropdownMenuSubContent across provider, Ollama, and LM Studio menus to
keep the list usable.

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