feat: add --mode uuid|word to random alias creation. Closes #51

[mexcool]

Summary

• Adds --mode flag (uuid or word) to sl alias create --random, forwarded as ?mode= query parameter to the SimpleLogin POST /api/alias/random/new endpoint.
• Validates the flag value and includes help text examples.
• Adds a dedicated test (TestClient_CreateRandomAlias_WithMode) verifying the query parameter is sent correctly.

Changed files

• internal/api/client.go — CreateRandomAlias now accepts a mode parameter and appends it to the URL.
• cmd/alias/create.go — new --mode flag with validation; passed through on the --random path only.
• internal/api/client_test.go — updated existing test call site; added new test for mode query parameter.

Test plan

• go build ./... passes
• go vet ./... passes
• go test ./... passes (including new TestClient_CreateRandomAlias_WithMode)