feat(settings): validate BaseUrl + cron on save, add BaseUrl hint

- Reject a non-absolute / non-http(s) BaseUrl and an implausible (not 5- or
  6-field) cron expression before the section is written to disk, mirroring the
  existing storage-path validation (snackbar + early return).
- Add a hint to the BaseUrl field. Cron check is a lightweight UI guard; the
  worker still does the authoritative Cronos parse at startup.

src/Marathon.UI/Pages/Settings.razor

@@ -49,7 +49,7 @@
             <Field Label="@L["Settings.Scraping.RetryBaseDelayMs"]">
                 <MudNumericField T="int" @bind-Value="_scraping.RetryPolicy.BaseDelayMs" Min="100" Max="60000" Variant="Variant.Outlined" />
             </Field>
-            <Field Label="@L["Settings.Scraping.BaseUrl"]">
+            <Field Label="@L["Settings.Scraping.BaseUrl"]" Hint="@L["Settings.Scraping.BaseUrl.Hint"]">
                 <MudTextField T="string" @bind-Value="_scraping.BaseUrl" Variant="Variant.Outlined" />
             </Field>
             <Field Label="@L["Settings.Scraping.RequestTimeoutSeconds"]">
@@ -242,6 +242,20 @@
             }
         }
 
+        if (payload is ScrapingSettingsForm scraping
+            && !(Uri.TryCreate(scraping.BaseUrl, UriKind.Absolute, out var baseUri)
+                 && (baseUri.Scheme == Uri.UriSchemeHttp || baseUri.Scheme == Uri.UriSchemeHttps)))
+        {
+            Snackbar.Add(L["Settings.Scraping.BaseUrl.Invalid"], Severity.Error);
+            return;
+        }
+
+        if (payload is WorkerOptions workers && !IsPlausibleCron(workers.UpcomingScheduleCron))
+        {
+            Snackbar.Add(L["Settings.Workers.Cron.Invalid"], Severity.Error);
+            return;
+        }
+
         var confirmed = await ConfirmAsync();
         if (!confirmed)
         {
@@ -260,6 +274,15 @@
         }
     }
 
+    // Lightweight 5- or 6-field cron sanity check — avoids a Cronos dependency in the
+    // UI layer; the worker still does the authoritative parse at startup.
+    private static bool IsPlausibleCron(string? expression)
+    {
+        if (string.IsNullOrWhiteSpace(expression)) return false;
+        var fields = expression.Split(' ', StringSplitOptions.RemoveEmptyEntries);
+        return fields.Length is 5 or 6;
+    }
+
     private async Task ResetSectionAsync(string section)
     {
         var confirmed = await ConfirmAsync();