revert: restore AP-mode grace period to 90s (3 checks)

The counter reset after NM restart already fully prevents the SSH-lockout
cascade: _disconnected_checks can never accumulate across NM restarts
because it is reset to 0 before the next daemon iteration runs.

The 3→6 increase provided no additional fix for the described problem and
caused a UX regression: fresh Pi devices with no WiFi configured would
wait 3 minutes instead of 90 seconds for the LEDMatrix-Setup hotspot to
appear.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/wifi_manager.py

@@ -140,7 +140,7 @@ class WiFiManager:
         # Initialize disconnected check counter for grace period
         # This prevents AP mode from enabling on transient network hiccups
         self._disconnected_checks = 0
-        self._disconnected_checks_required = 6  # Require 6 consecutive disconnected checks (3 minutes at 30s interval)
+        self._disconnected_checks_required = 3  # Require 3 consecutive disconnected checks (90 seconds at 30s interval)
 
         # Timestamp set when AP mode is enabled; used for the idle-timeout check
         self._ap_enabled_at: Optional[float] = None