pi-extension: make startup notification opt-out via quietStartup (#308)

The status bar already shows the active ponytail mode at all times, but the
startup toast is also a deliberate trust-heartbeat for a behavior-modifying
extension (it confirms ponytail loaded, in which mode, and survives session
resume where the persisted mode may differ from the default). Default
behavior is preserved.

Users who prefer a clean startup can now silence it via:

  - config:  ~/.config/ponytail/config.json -> { "quietStartup": true }
  - env var: PONYTAIL_QUIET_STARTUP=1

Resolution order mirrors the existing defaultMode setting: env var > config
file > default (false). Added readQuietStartup export and a test covering
all three layers.

Co-authored-by: blissini <blissini@users.noreply.github.com>
Co-authored-by: Emeriko <dietrich.gebert@gmail.com>