perf: optimize model dialog visibility lookups

[shkumbinhasani]

Summary

• Fixes model dialog lag that occurs during longer sessions
• Replaces O(n) array lookups with O(1) Set/Map lookups in the visible() function

Problem

The visible() function in packages/app/src/context/local.tsx was called inside a .filter() loop for each model. Each call performed:

1. A linear search through store.user array
2. A linear search through latest() array (which involves expensive date computations)

With many models, this became O(n²) and caused noticeable lag when opening the model selector.

Solution

Added two memoized lookup structures:

• latestSet: A Set for O(1) lookup of whether a model is "latest"
• userVisibilityMap: A Map for O(1) lookup of user visibility preferences

The memos are only recomputed when their underlying data changes, not on every visible() call.