fix(ui): validate PowerUse window, explain effort and Prime Claude in Settings

Three usability-audit findings in Settings:
- An unparseable PowerUse window (e.g. "9:0") used to save silently as null
  (= round the clock), the opposite of what the typo implied. Validate() now
  rejects an unparseable start/end unless blank; the worker-side read-time
  tolerance is unchanged.
- "Effort" column header gets a tooltip explaining it's the model's reasoning
  effort (cost/depth tradeoff).
- The "Prime Claude" tab gets a tooltip explaining scheduled Claude runs
  (Ping / Fill My Day / custom prompt).

New locale keys added to en+de. Updated the PowerUse round-trip test that
asserted the old silent-normalize behavior.

src/ClaudeDo.Ui/ViewModels/Modals/Settings/GeneralSettingsTabViewModel.cs

@@ -119,8 +119,13 @@ public sealed partial class GeneralSettingsTabViewModel : ViewModelBase
         foreach (var row in ModelPresets)
             if (row.MaxTurns is < 1 or > 200)
                 return $"Max turns for {row.Model} must be between 1 and 200.";
+        if (!IsValidWindowBound(PowerUseWindowStart) || !IsValidWindowBound(PowerUseWindowEnd))
+            return Loc.T("settings.general.powerUseWindowInvalid");
         return null;
     }
+
+    private static bool IsValidWindowBound(string? value)
+        => string.IsNullOrWhiteSpace(value) || PowerUseWindow.Parse(value) is not null;
 }
 
 /// <summary>One row of the per-model preset table. Single consumer, so it lives here.</summary>