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>

web_interface/blueprints/api_v3.py

@@ -4,6 +4,7 @@ import os
 import re
 import stat
 import sys
+import shutil
 import subprocess
 import tempfile
 import time
@@ -25,6 +26,9 @@ from src.web_interface.validators import (
 )
 from src.error_aggregator import get_error_aggregator
 
+_SUDO = shutil.which('sudo')
+_JOURNALCTL = shutil.which('journalctl')
+
 # Will be initialized when blueprint is registered
 config_manager = None
 plugin_manager = None
@@ -1459,10 +1463,16 @@ def execute_system_action():
                 result = subprocess.run(['sudo', 'systemctl', 'start', 'ledmatrix'],
                                      capture_output=True, text=True)
                 logger.info("start_display (%s) returned code %d", mode, result.returncode)
-                return jsonify({
+                if result.returncode != 0 and result.stderr:
+                    logger.error("start_display (%s) stderr: %s", mode, result.stderr.strip())
+                resp = {
                     'status': 'success' if result.returncode == 0 else 'error',
                     'message': 'Display started' if result.returncode == 0 else 'Failed to start display',
-                })
+                }
+                if result.returncode != 0:
+                    resp['returncode'] = result.returncode
+                    resp['stderr'] = result.stderr.strip()
+                return jsonify(resp)
             else:
                 result = subprocess.run(['sudo', 'systemctl', 'start', 'ledmatrix'],
                                      capture_output=True, text=True)
@@ -1564,10 +1574,16 @@ def execute_system_action():
             return jsonify({'status': 'error', 'message': 'Unknown action'}), 400
 
         logger.info("system action '%s' returncode=%d", action, result.returncode)
-        return jsonify({
+        if result.returncode != 0 and result.stderr:
+            logger.error("system action '%s' stderr: %s", action, result.stderr.strip())
+        resp = {
             'status': 'success' if result.returncode == 0 else 'error',
             'message': 'Action completed' if result.returncode == 0 else 'Action failed; check logs for details',
-        })
+        }
+        if result.returncode != 0:
+            resp['returncode'] = result.returncode
+            resp['stderr'] = result.stderr.strip()
+        return jsonify(resp)
 
     except Exception as e:
         logger.error("execute_system_action failed: %s", e, exc_info=True)
@@ -6425,9 +6441,12 @@ def list_plugin_assets():
 def get_logs():
     """Get system logs from journalctl"""
     try:
+        if not _SUDO or not _JOURNALCTL:
+            return jsonify({'status': 'error', 'message': 'sudo or journalctl not found on this system'}), 503
         # Get recent logs from journalctl
         result = subprocess.run(
-            ['sudo', 'journalctl', '-u', 'ledmatrix.service', '-n', '100', '--no-pager'],
+            [_SUDO, _JOURNALCTL, '-u', 'ledmatrix.service', '-u', 'ledmatrix-web.service',
+             '-n', '100', '--no-pager', '--output=short-iso'],
             capture_output=True,
             text=True,
             timeout=5
@@ -6438,7 +6457,7 @@ def get_logs():
             return jsonify({
                 'status': 'success',
                 'data': {
-                    'logs': logs_text if logs_text else 'No logs available from ledmatrix service'
+                    'logs': logs_text if logs_text else 'No logs available from ledmatrix or ledmatrix-web service'
                 }
             })
         else: