[cleanup] Phase 5: Dependency deduplication

## Summary
Address duplicate dependencies and remove mock libraries.

## Actions
- [ ] Investigate schemars 0.8/1.2 duplication
- [ ] Upgrade dashmap to 6.x workspace-wide
- [ ] Address reqwest 0.11/0.12 split
- [ ] Remove mockall from desktop/src-tauri/Cargo.toml

## Verification
- `cargo tree -d` shows reduced duplicates
- `cargo build --workspace` passes
- `cargo test --workspace` passes