Add device-aware CLI, devices command, completion scripts, and connect wait/retry

[shazow]

Motivation

• Provide a way to target a specific wireless interface from the CLI and surface available interfaces for scripting and shell completion.
• Let connect optionally wait for an SSID to appear and retry until timeout so non-interactive scripts can robustly connect to networks that appear shortly after invocation.
• Expose device enumeration in the backend abstraction so platform implementations (NetworkManager/iwd/darwin) and the mock backend can expose and be tested for device selection.

Description

• Add a global --device option (in Options) and call configureBackendDevice() from each command handler to apply the selected device via Backend.SetDevice before operations.
• Extend the backend API with a Device model and two methods: ListDevices() ([]Device, error) and SetDevice(name string) error in wifi/backend.go, and implement them in wifi/networkmanager, wifi/iwd, wifi/darwin, and wifi/mock.
• Implement wifitui devices (runDevices) that by default prints stable, line-separated device names and supports --json to emit richer metadata such as name, type, state, backend, and optional details.
• Add shell completion generation (wifitui completion <bash|zsh|fish>) and completed scripts that dynamically source --device values from wifitui devices and SSID suggestions for connect from wifitui list --all --scan.
• Add connect retry/wait behavior: new --retry parsing (DURATION[:INTERVAL]) and --wait to enable "require visible" semantics so connect will rescan and retry until the SSID appears or timeout; updated runConnect/attemptConnect to emit visibility-specific retry messages.
• Update mock backend to expose multiple devices and implement ListDevices/SetDevice for deterministic tests, and adjust platform backends to enumerate/select devices where applicable.
• Update CLI parsing and defaults (main.go / cli.go) and add new/changed tests for CLI parsing, device routing, device command behavior, and connect retry/wait semantics in cli_test.go and wifi/mock/mock_test.go, and document usage examples in README.md.

Testing

• Updated and added unit tests were run with go test ./... and all tests passed successfully.
• New/changed tests include CLI parsing and behavior (cli_test.go) and mock backend device tests (wifi/mock/mock_test.go) and exercised connect retry/wait behavior with simulated backends.
• Manual validation was performed by running the generated shell completion outputs and the new devices and connect --wait flows in tests (captured by the automated tests above).

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