ui : model memory-fit estimation (#27957)

* ui : model memory-fit estimation

Replace the raw runtime-memory estimate with the app's compatibility check:
the smallest real Mac memory tier that fits a model file, budgeted as
RAM x 0.75 minus fixed overhead with headroom on the file size. The constants
move to lib; the unused runtime-memory estimate is dropped. browser-info's
OS detection is exported for reuse.

Assisted-by: pi:GLM-5.3-Flash

* ui : cover the memory-fit and tool-use heuristics in tests

Assisted-by: pi:zai-org/GLM-5.3-Flash