[cleanup] Phase 5: Dependency deduplication

[AlexMikhalev]

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

[AlexMikhalev]

PR #634 created: removed unused mockall from desktop/src-tauri, documented that most dependency duplicates are transitive/unavoidable.