feat(app): remove downloaded models safely

[PhilipJohnBasile]

Fixes #374.

The desktop model picker now offers a confirmed Remove action for installed models. Removal delegates to the existing mtplx remove JSON contract, then refreshes the picker and model-update state.

Safety boundaries:

• only direct children of the managed MTPLX cache are removable
• the selected model and active model transfers are protected
• user-managed folders and cached symlinks do not get the app removal action
• explicit CLI removal unlinks a cache symlink without following or deleting its external target
• unexpected non-directory cache entries fail closed

The CLI removal command is now included in the README quick reference.

Validation:

• full Python suite: exit 0, 4,377 tests collected
• full Swift suite: 659 passed, 1 existing skip
• focused removal tests: 34 Python and 6 Swift passed
• real CLI temporary-cache removal smoke: passed
• swift build, Ruff, and git diff --check: passed

@youssofal, please review.