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);
+    }
 }

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

@@ -153,10 +153,17 @@ public class SettingsModalViewModelTests : IDisposable
         Assert.Equal("18:00", vm.General.PowerUseWindowStart);
         Assert.Equal(88, vm.General.PowerUseHandoffPct);
 
-        // A typo in the window is normalized to null on save, not stored as a fake window.
+        // A typo in the window blocks Save instead of silently becoming "round the clock".
         vm.General.PowerUseWindowEnd = "not a time";
         await vm.SaveCommand.ExecuteAsync(null);
 
+        Assert.Null(worker.Saved);
+        Assert.NotEqual("", vm.ValidationError);
+
+        // Clearing it back out is still a valid "round the clock" window and saves.
+        vm.General.PowerUseWindowEnd = "";
+        await vm.SaveCommand.ExecuteAsync(null);
+
         Assert.True(worker.Saved!.PowerUseEnabled);
         Assert.Equal("18:00", worker.Saved.PowerUseWindowStart);
         Assert.Null(worker.Saved.PowerUseWindowEnd);