feat(cron): monitor-mode jobs (monitor_script/monitor_url) + blocked_config status (issue #861) (#871)

- CronJob model gains optional monitor_script/monitor_url (additive-safe)
- Job editor: Monitor mode section (off/script/url) with XOR source clearing;
  disabled while no-agent is set (backend invariant); new jobs apply monitor
  via create -> follow-up PUT with rollback delete on failure (the REST create
  model drops the fields)
- Job list/details: blocked_config renders as a distinct error badge
  ('Blocked (config)'), no_change as informational; monitor source shown in
  run details

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/ui/common/ExposedDropdownField.kt

@@ -30,17 +30,19 @@ fun ExposedDropdownField(
     options: List<String>,
     selectedValue: String,
     onOptionSelected: (String) -> Unit,
+    enabled: Boolean = true,
 ) {
     var expanded by remember { mutableStateOf(false) }
 
     ExposedDropdownMenuBox(
         expanded = expanded,
-        onExpandedChange = { expanded = it },
+        onExpandedChange = { if (enabled) expanded = it },
     ) {
         OutlinedTextField(
             value = selectedValue.ifEmpty { options.firstOrNull() ?: "" },
             onValueChange = {},
             readOnly = true,
+            enabled = enabled,
             modifier =
                 Modifier
                     .fillMaxWidth()