feat: dashboard-only mode and keystone settings integration

The second display is now optional: [display] enabled = false skips the
headless output, wayvnc, AVNC and the VNC tunnel, so the tablet runs the
dashboard alone. `deck-it env` reports it as EXTEND.

Keystone's new deck-it page (quickshell, desktop/modules/deckit) edits
the config and the autostart line, which needed:

- The user config is merged over the shipped config.toml key by key
  within each table instead of replacing it whole, so the page writes a
  sparse file and an unset key falls back to the default.
- `deck-it defaults` prints the shipped config as JSON for the page's
  placeholders, so keystone never copies the defaults.
- `deck-it status` prints one JSON line (server, display, waiting,
  tablet, extend) for the page's status line and restart check.
- [launch] is now `apps`, a list of desktop ids run with gtk-launch,
  with name and icon read from the .desktop file, so the page can pick
  launchers from the installed apps. The old command tables are gone;
  the client still sends only an id, which must be listed and installed.
  A launcher whose icon the theme lacks shows its name.
- [wifi] address gives the tablet's address in config. The last address
  that worked is kept in ~/.config/deck-it/last-wifi: `stop` deletes the
  runtime serial file, so a restart from the page used to wait forever.
- `stop` also ends a login's `wifi --wait`, matched by its own command
  line only, so repeated restarts do not pile up waiters.

Also fixed: `deck-it env` fell through into the usage text, and the
usage text printed the license header since the header was added.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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