fix(logs): include ledmatrix-web logs in viewer and log subprocess stderr on failure

Two bugs conspired to produce "check the logs" toasts with an empty log viewer:

1. The log viewer (both SSE stream and REST endpoint) only queried
   ledmatrix.service via journalctl. Web API errors are logged by the
   Flask process running as ledmatrix-web.service, so they never
   appeared in the viewer. Add -u ledmatrix-web.service to both calls;
   also add --output=short-iso so timestamps from the two services
   sort cleanly when interleaved. Use shutil.which-resolved absolute
   paths for sudo/journalctl (S607 compliance) in api_v3.py; fall back
   to known Pi paths if which returns None.

2. app.py: resolve journalctl and systemctl to absolute paths via
   shutil.which at module init (_JOURNALCTL, _SYSTEMCTL). Replace bare
   names in logs_generator() and the cached systemctl is-active check.
   Guard both sites: logs_generator yields a clear SSE error message
   and sleeps 60 s if journalctl is not found; the systemctl block is
   skipped entirely if systemctl is not found, leaving the cache at its
   last-known value.

3. When execute_system_action() ran a systemctl command that returned
   non-zero, only the return code was logged — result.stderr was
   silently discarded. Log it at ERROR level and include returncode and
   stderr in the JSON response so callers get actionable failure details.
   Same fix applied to the early-return start_display branch.

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

src/plugin_system/state_reconciliation.py

@@ -185,19 +185,13 @@ class StateReconciliation:
                 message=f"Reconciliation failed: {str(e)}"
             )
     
-    # Top-level config keys that are NOT plugins.
+    # Top-level config keys that are NOT plugins
-    # Includes both config.json structural keys and config_secrets.json top-level
-    # keys (load_config() deep-merges secrets in, so secrets keys appear here too).
     _SYSTEM_CONFIG_KEYS = frozenset({
         'web_display_autostart', 'timezone', 'location', 'display',
         'plugin_system', 'vegas_scroll_speed', 'vegas_separator_width',
         'vegas_target_fps', 'vegas_buffer_ahead', 'vegas_plugin_order',
         'vegas_excluded_plugins', 'vegas_scroll_enabled', 'logging',
         'dim_schedule', 'network', 'system', 'schedule',
-        # Multi-display sync config (config.json structural key)
-        'sync',
-        # Secrets file top-level keys (merged in by load_config)
-        'github', 'youtube',
     })
 
     def _get_config_state(self) -> Dict[str, Dict[str, Any]]: