fix(instances): correct stopped VM resource reporting and add tests

[laolusrael]

Summary

Fixes the endpoint /api/instances/{name} returning incorrect resource values (memory: "0 B", disk: "0 B") for stopped instances.

Problem

When a VM is stopped, multipass info reports memory.total: 0 and disk.total: 0. The existing fallback logic checked instance.Memory == "" and instance.Disk == "", but formatBytes(0) returns "0 B" -- so the fallback to multipass get never triggered.

Fix

• Check the raw memBytes and diskBytes values (before formatting) for zero instead of checking the formatted strings
• This correctly triggers the fallback to multipass get local.<name>.{cpus,memory,disk} for stopped VMs

Also Includes

• Linux disk detection improvements -- GetHostInfo() uses multipassDataDir() to locate VM data directory
• macOS data directory detection -- added Library/Application Support/multipassd paths
• Handler tests -- 8 UpdateResources tests + HostHandler tests
• Mock client fix -- GetInstance returns proper error instead of nil, nil
• Integration test fix -- skips when multipass is unavailable

Test Results

• go test -tags ci ./... -- Pass
• gofmt -d . -- Clean
• npx vitest --run -- 15/15 pass
• npm run lint -- Clean
• npm run build -- Success