feat(ui): add usage monitor modal with gauges and model/task usage analysis

Adds a Usage Monitor modal (Worker menu + wired to the footer/Mission-Control
usage pill's Open command): dynamic gauges built from UsageSnapshotDto.Limits
with gate-threshold marks, a stale/blocked-gate band, and Models/Tasks tabs
backed by GetModelUsageAsync/GetTaskUsageAsync over a 7d/30d/custom range.

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -66,6 +66,8 @@
                         Command="{Binding RestartWorkerCommand}"/>
               <MenuItem Header="{loc:Tr shell.menu.checkForUpdates}"
                         Command="{Binding CheckForUpdatesCommand}"/>
+              <MenuItem Header="{loc:Tr shell.menu.usageMonitor}"
+                        Command="{Binding OpenUsageMonitorCommand}"/>
             </MenuItem>
             <MenuItem Header="{loc:Tr shell.menu.repositories}"
                       FontSize="{StaticResource FontSizeMono}"