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/Services/UsageTimeFormat.cs

@@ -0,0 +1,19 @@
+using System;
+using ClaudeDo.Ui.Localization;
+
+namespace ClaudeDo.Ui.Services;
+
+/// <summary>Shared "Xh Ym" formatting for usage-reset countdowns (pill tooltip + usage monitor gauges).</summary>
+public static class UsageTimeFormat
+{
+    public 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);
+    }
+}