Fix desktop chrome and bound model scans

[timtoole02]

Summary

• restore the native desktop title bar and remove the unreliable overlay-title-bar detection, drag strip, and styling
• add a generic macOS removable-volume usage description for user-selected external model storage
• move local GGUF metadata parsing off the async request worker and bound each scan to five seconds

Why

The overlay title bar could leave the macOS desktop window difficult to drag. Native window chrome is simpler and more reliable.

Model metadata reads can block on slow, disconnected, or permission-gated removable storage. The bounded blocking task keeps the local-models API responsive and reports an actionable per-file admission reason instead of hanging the request.

User impact

Desktop users get reliable native window controls. Users who keep models on external drives receive the appropriate macOS permission prompt, and an unavailable drive can no longer indefinitely block model discovery.

This PR is based directly on main and has no dependency on PR #644.

Validation

• cargo fmt --all -- --check
• cargo check --locked --bin camelid
• focused local-model scan test: 1 passed
• cargo test --locked -p camelid-desktop: 22 passed
• npm.cmd run build
• git diff --check