refactor(ui): dedupe FormatRemaining between UsagePill and UsageMonitorModal

Both viewmodels carried byte-identical copies of the reset-countdown
formatter. Moved it to UsageTimeFormat.FormatRemaining and pointed
both call sites at it; locale keys and output unchanged.

src/ClaudeDo.Ui/ViewModels/Modals/UsageMonitorModalViewModel.cs

@@ -391,8 +391,10 @@ public sealed partial class UsageGaugeRowViewModel : ObservableObject
 
     public bool IsWarnSeverity => !string.Equals(Severity, "normal", StringComparison.OrdinalIgnoreCase);
 
+    // Includes the absolute HH:mm alongside the countdown, unlike the pill tooltip's shorter
+    // "resetIn" line (usage.pill.resetIn) — the modal has room for the detail, the tooltip doesn't.
     public string ResetText => ResetsAt is { } r
-        ? Loc.T("modals.usageMonitor.resetIn", FormatRemaining(r), r.ToLocalTime().ToString("HH:mm"))
+        ? Loc.T("modals.usageMonitor.resetIn", UsageTimeFormat.FormatRemaining(r), r.ToLocalTime().ToString("HH:mm"))
         : "";
 
     /// <summary>Live values from a fresh snapshot, without replacing the row instance mid-view.</summary>
@@ -437,17 +439,6 @@ public sealed partial class UsageGaugeRowViewModel : ObservableObject
 
         return Commit();
     }
-
-    private static string FormatRemaining(DateTimeOffset resetsAt)
-    {
-        var remaining = resetsAt - DateTimeOffset.UtcNow;
-        if (remaining < TimeSpan.Zero) remaining = TimeSpan.Zero;
-        var hours = (int)remaining.TotalHours;
-        var minutes = remaining.Minutes;
-        return hours > 0
-            ? Loc.T("usage.pill.durationHoursMinutes", hours, minutes)
-            : Loc.T("usage.pill.durationMinutes", minutes);
-    }
 }
 
 /// <summary>