fix: service control buttons and AP-mode SSH lockout post-install

Two user-reported issues after fresh install:

1. All service buttons (Start/Stop/Restart Display, Restart Web Service)
   failed silently — only Reboot worked.

   Root cause: sudoers rules use `ledmatrix.service` (with suffix) but
   api_v3.py called `sudo systemctl start ledmatrix` (no suffix). sudo
   does exact string matching, so every service action was rejected with
   returncode=1. Also missing from sudoers: ledmatrix-web, journalctl,
   and is-active entries.

   Fix:
   - Add `.service` suffix to all 8 sudo systemctl call sites in
     api_v3.py (_ensure_display_service_running, _stop_display_service,
     and all execute_system_action branches).
   - Add timeout=15 to all subprocess.run calls in execute_system_action
     (previously could hang indefinitely).
   - Add missing sudoers rules to first_time_install.sh and
     configure_web_sudo.sh: ledmatrix-web.service start/stop/restart,
     is-active for both name forms, and journalctl -u/-t ledmatrix rules.

2. SSH and web UI became inaccessible after ~1 hour even though the
   display kept running.

   Root cause: wifi_monitor_daemon restarts NetworkManager after 5
   consecutive internet failures (~2.5 min). Each NM restart drops WiFi
   briefly. During that window check_and_manage_ap_mode() increments
   _disconnected_checks but the daemon never reset it after the restart.
   After 3 such NM-restart cycles, _disconnected_checks reached 3 and
   AP mode activated — changing the Pi from WiFi client to hotspot
   (192.168.4.1) and killing SSH on the old IP.

   Fix:
   - Reset wifi_manager._disconnected_checks = 0 in the daemon
     immediately after a successful NM restart so the brief drop it
     causes doesn't count toward AP-mode activation.
   - Increase _disconnected_checks_required from 3 to 6 (90s → 3min)
     as an additional buffer against transient network flaps.

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 = 3  # Require 3 consecutive disconnected checks (90 seconds at 30s interval)
+        self._disconnected_checks_required = 6  # Require 6 consecutive disconnected checks (3 minutes at 30s interval)
 
         # Timestamp set when AP mode is enabled; used for the idle-timeout check
         self._ap_enabled_at: Optional[float] = None