feat(cli): introduce headless amnezia-cli for desktop automation

[vitkuz573]

Why this PR exists

Amnezia had desktop flows and partial command handling, but not a true headless client suitable for terminal-first usage and automation.
This PR adds a dedicated CLI binary that works with the same core models/controllers as the app, so behavior stays consistent.

What this PR adds

• A new executable: amnezia-cli (client/cli)
• Full command surface for day-to-day operations:
  • VPN state and control: status, connect, disconnect
  • Daemon lifecycle: daemon start|stop|status
  • Server management: servers list|show|add|import|remove|set-default|scan
  • Country selection for API-backed configs: countries list|set
  • Container management: containers list|set-default|remove
  • Provisioning flows: install server, install container
  • Maintenance: logs cleanup
• CLI documentation: client/cli/README.md

Implementation highlights

• Added a separate CMake target for CLI (client/cli/CMakeLists.txt) and wired it into the client build graph.
• Reworked several controller access paths to be safe in headless context:
  • network manager access no longer depends on UI-only app helpers
  • clipboard operations are guarded for GUI availability
• Result: shared business logic can be used both by desktop UI and by CLI without duplicating behavior.

Verification

• cmake --build build --target amnezia-cli -j4
• ./build/client/cli/amnezia-cli --help
• ./build/client/cli/amnezia-cli status --json
• ./build/client/cli/amnezia-cli servers list --json
• ./build/client/cli/amnezia-cli countries list --index 0 --json

[WkdXeqtr]

I've been looking for something like this for a while. Having a CLI to connect/disconnect, switch servers, and control everything programmatically makes Amnezia actually usable in scripts and automation setups. Really hoping this gets some attention from maintainers - any chance of a review?