Enable an offline mode that passes `--offline` to cargo

When working in slow or no internet situations, RA seems to lock the registry/workspace trying to do internet things. It can be tricky to cancel these operations.

My suggestion is to add a new offline mode that can be toggled via the menu. This can either set the environment variable (CARGO_NET_OFFLINE=true) or pass --offline to each cargo command (I'd prefer this so it's easy to tell the mode is being used). This would be two new menu options:

• "rust-analyzer: Enable offline mode": cancel any running operations, set the flag, and relaunch anything in progress
• "rust-analyzer: Disable offline mode": allow running operations to complete, remove --offline flag

Start by tracing how rust-analyzer exposes menu options and launches cargo commands. Determine where running operations can be cancelled or relaunched, then implement enable and disable offline actions that apply or remove --offline. Done means the mode is visible and toggleable, active operations follow the requested behavior, and cargo commands run without network access when enabled.