Add per-tab option to set a default input channel without locking it

[Fasted93]

Right now a tab's "Input channel" is enforced every frame and the channel switcher is disabled. With no input channel set, the tab just inherits whatever channel the game picks on login, which is usually Novice Network.

This adds a "Lock input channel" checkbox per tab. It defaults to on, so existing configs behave exactly as before.

When unchecked, the selected channel is applied when switching to the tab and a few seconds after login, but you can still change the channel afterwards through the switcher, keybinds or commands.

Changes:

• New LockChannel field on Tab (default true, no config migration needed)
• Skip the per-frame channel enforcement and enable the switcher when the tab is unlocked
• Apply the tab's channel shortly after login, since the game switches to Novice Network a moment after the Login event. I used a 3 second delay; if that turns out to be too short on some setups, bumping it to 5 should be safe
• One new localization string