feat(app): favorite models for quick switching in model selector

Problem

When working with multiple projects or different task complexities, I frequently switch between 3-4 models (e.g. deepseek-v4-pro for complex work, gemma-4 for simple fixes, claude-sonnet for architecture reviews). Currently this requires opening the full model selector popover, scrolling/searching each time. With ~100 visible models across providers, the 2-3 models I actually use are buried in the list.

Proposed solution

Favorite models quick-switch in the model selector popover (ModelSelectorPopoverV2 in packages/app/src/components/dialog-select-model.tsx):

1. A ★ star icon next to each model in the list — click to toggle favorite state (persisted in localStorage)
2. A Favorites section pinned at the top of the popover (above search) — pill-shaped buttons for each favorited model, one-click select
3. Favorite models are always sorted first in the main list

This is a small, focused UI change (~50-70 lines) that doesn't alter any core routing or model selection logic. It reuses existing useLocal().model.set() and the popover's close mechanism.

Why belongs in OpenCode

The model selector is the most-used UI element for users who routinely switch models. A favorites system reduces friction without adding complexity — it's a standard pattern (browser bookmarks, IDE favorites) that users already expect.