fix: address five review findings (NM retry loop, start_display message, code quality)

- wifi_monitor_daemon: reset _consecutive_internet_failures = 0 in both
  NM-restart exception handlers; previously both left the counter at threshold,
  causing an immediate retry on the next iteration instead of waiting another
  full backoff period

- api_v3: fix start_display failure message — when mode is set and systemctl
  returns non-zero, message now includes the failure reason and a hint rather
  than always reporting success phrasing

- wifi_manager: move _redirect_backend from class variable to instance variable
  in __init__ alongside _ap_enabled_at; class-level default shadowed correctly
  in practice (single instance) but was misleading

- wifi_manager: narrow broad except Exception in _check_internet_connectivity
  to (subprocess.SubprocessError, OSError) for ping and OSError for HTTP
  (urllib.error.URLError is an OSError subclass in Python 3)

- wifi_manager: remove redundant local 'import re as _re' in _validate_ap_config;
  re is already imported at module level (line 37)

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

web_interface/blueprints/api_v3.py

@@ -1720,7 +1720,8 @@ def execute_system_action():
                                      capture_output=True, text=True, timeout=15)
                 return jsonify({
                     'status': 'success' if result.returncode == 0 else 'error',
-                    'message': f'Started display in {mode} mode',
+                    'message': f'Started display in {mode} mode' if result.returncode == 0
+                               else f'Failed to start display in {mode} mode: {result.stderr.strip() or "check sudo systemctl status ledmatrix.service"}',
                     'returncode': result.returncode,
                     'stdout': result.stdout,
                     'stderr': result.stderr