Add headless CLI for uploading/downloading keyboard configurations

[joshperry]

Adds vial-cli, a headless command-line interface for writing and reading .vil layout files without launching the GUI. This enables scripted keyboard configuration, LLM MCP, remote/SSH workflows, and CI-driven provisioning on systems without a display server.

Usage

vial-cli upload <file.vil>      # write a saved layout to the keyboard
vial-cli download <file.vil>    # read the current layout from the keyboard
vial-cli upload layout.vil --device 1  # select keyboard when multiple are connected
vial-cli upload layout.vil --force     # skip UID mismatch check

How it works

This builds on the protocol/GUI decoupling from #382 (deguify) — since the protocol layer no longer imports Qt at module level, the CLI can drive Keyboard.save_layout() and Keyboard.restore_layout() directly over USB without any display dependencies.

Two things that still needed Qt are handled at runtime:

• Unlocker — a headless implementation is injected into sys.modules before any protocol code runs. It prints the key positions to hold and polls in a text loop instead of showing the Qt dialog.
• QmkSettings — initialized by loading qmk_settings.json directly, bypassing the GUI's appctx-based resource resolution.

Device discovery

When zero or multiple keyboards are detected, the CLI prints actionable diagnostics (troubleshooting hints for zero, a numbered device list for multiple) and exits non-zero.

UID matching

.vil files store keyboard UIDs as JSON numbers, which lose precision through float64 when created by JavaScript-based tools (e.g. the web configurator). The CLI compares UIDs through float() to tolerate this, and supports --force as an explicit override.

Tests

20 new tests in test_cli.py covering device discovery (all 6 branches), upload validation, the float64 UID edge case, headless unlock flow, download round-trip, and an AST check that cli.py remains Qt-free.

[Pituivan]

This is actually really useful. For some reason, my keyboard's persistent memory corrupts when rebooting (I think it's because I use a cheap USB hud), so I have to manually open Vial and load my last saved layout.

This PR would allow me to automate that process on startup.