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.

tests/ClaudeDo.Ui.Tests/ViewModels/GeneralSettingsTabViewModelTests.cs

@@ -30,4 +30,31 @@ public class GeneralSettingsTabViewModelTests
 
         Assert.Equal(expectedValid, vm.Validate() is null);
     }
+
+    [Theory]
+    [InlineData(null, true)]
+    [InlineData("", true)]
+    [InlineData("09:00", true)]
+    [InlineData("23:59", true)]
+    [InlineData("9:0", false)]
+    [InlineData("abc", false)]
+    [InlineData("25:00", false)]
+    public void Validate_PowerUseWindowStart_rejectsUnparseable(string? value, bool expectedValid)
+    {
+        var vm = new GeneralSettingsTabViewModel { PowerUseWindowStart = value };
+
+        Assert.Equal(expectedValid, vm.Validate() is null);
+    }
+
+    [Theory]
+    [InlineData(null, true)]
+    [InlineData("09:00", true)]
+    [InlineData("9:0", false)]
+    [InlineData("abc", false)]
+    public void Validate_PowerUseWindowEnd_rejectsUnparseable(string? value, bool expectedValid)
+    {
+        var vm = new GeneralSettingsTabViewModel { PowerUseWindowEnd = value };
+
+        Assert.Equal(expectedValid, vm.Validate() is null);
+    }
 }