Universal Spell Check v0.1.1

feat: launch on Windows startup by default, toggleable in Settings

Velopack does not register a login auto-start hook and our code
didn't either, so v0.1.0 only ran when launched manually. The app
is meant to be ambient — you press Ctrl+Alt+U and it Just Works —
which only holds if it's already running. Default to enabled, but
let users opt out from the dashboard.

New StartupRegistration writes HKCU\...\Run\<MutexName> pointing at
the running exe (the channel-specific mutex name keeps Prod and Dev
from clobbering each other). On first launch after install,
SpellCheckAppContext calls EnsureFirstRunRegistered which registers
the entry and drops a startup.initialized flag in the per-channel
AppData folder. The flag is what stops us from re-enabling the
entry on every launch after a user has explicitly disabled it.

The Settings page already had a Startup card with a disabled
checkbox. Wire it up: the checkbox now reflects IsRegistered() on
load and toggling it calls Register/Unregister. In Dev the checkbox
is disabled with a tooltip explaining that dev builds are launched
manually via dotnet run, since the dev exe path moves with each
publish anyway.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>