First-launch setup and CLI app launcher

What's new

First-launch welcome screen. New users now see a setup screen on first launch where they pick between menu bar mode (the default) and dock/window mode. One click and you're in. Existing users who've already chosen a mode won't see it.
whatcable --desktop and whatcable --popover CLI flags. Launch the GUI app from the terminal in either mode. Sets the preference and opens the app. Handy if you installed via Homebrew and want to fire up the GUI without hunting for it in Finder.

Instructions page rewritten. The docs at whatcable.uk/instructions are now written for a broader audience. Getting started, common questions, and everyday features come first. Technical details are still there, just further down the page.
README updated to reflect the current language count, settings, CLI flags, and module structure.

brew upgrade --cask whatcable or download from Releases.