Add Locking UI

[bburda]

Summary

Add resource locking UI with acquire/release/extend lock functionality and a "locked" badge in the entity tree.

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Proposed solution (optional)

• Lock management panel per entity
• Acquire lock via POST /{entity_type}/{entity_id}/locks
• List locks via GET /{entity_type}/{entity_id}/locks
• Extend/release lock via PUT /{entity_type}/{entity_id}/locks/{lock_id}
• "Locked" badge/icon on locked entities in the tree view
• Show lock expiration timer

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Additional context (optional)

Depends on sovd_web_ui#42 (client migration). Gateway enforces locks on all mutating handlers (PUT, POST, DELETE) since v0.4.0.