feat: implement board tab with dialogs, virtualized card lists and refresh-after-write

- board: columns in id order, per-column sort via config, column width presets (240/320/480) and custom 160-720
- column header new-card and edit/delete menu; card menu delete and move-to-column
- card editor as one unit of work with dirty check and discard confirmation; nested tag picker writes new tags immediately
- schema constraint violations mapped to typed exceptions and shown inside the modal
- dialog service with owner stack so nested modals center on their parent and get FlowDirection
- repository: filtered GetCards, CountCards; view-model tests

YKanBan/Views/Dialogs/ColumnWidthDialog.axaml

@@ -0,0 +1,30 @@
+<dialogs:DialogWindow xmlns="https://github.com/avaloniaui"
+                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                      xmlns:vm="using:YKanBan.ViewModels.Dialogs"
+                      xmlns:dialogs="using:YKanBan.Views.Dialogs"
+                      xmlns:lang="using:YKanBan"
+                      x:Class="YKanBan.Views.Dialogs.ColumnWidthDialog"
+                      x:DataType="vm:ColumnWidthDialogViewModel"
+                      Title="{x:Static lang:Resources.ColumnWidth_Title}"
+                      Width="360"
+                      WindowStartupLocation="CenterOwner">
+
+    <StackPanel Margin="20" Spacing="10">
+        <RadioButton GroupName="Width" Content="{Binding NarrowLabel}" IsChecked="{Binding IsNarrow}"/>
+        <RadioButton GroupName="Width" Content="{Binding StandardLabel}" IsChecked="{Binding IsStandard}"/>
+        <RadioButton GroupName="Width" Content="{Binding WideLabel}" IsChecked="{Binding IsWide}"/>
+        <RadioButton GroupName="Width" Content="{x:Static lang:Resources.ColumnWidth_Custom}" IsChecked="{Binding IsCustom}"/>
+        <StackPanel Orientation="Horizontal" Spacing="8" Margin="28,0,0,0" IsEnabled="{Binding IsCustom}">
+            <NumericUpDown Value="{Binding CustomPixels}" Minimum="{Binding MinPixels}" Maximum="{Binding MaxPixels}"
+                           Increment="10" FormatString="0" Width="140"/>
+            <TextBlock Text="{Binding RangeLabel}" VerticalAlignment="Center" Opacity="0.7"/>
+        </StackPanel>
+
+        <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Spacing="8" Margin="0,8,0,0">
+            <Button Content="{x:Static lang:Resources.Common_Ok}" Command="{Binding ConfirmCommand}"
+                    Classes="Primary" MinWidth="90" IsDefault="True"/>
+            <Button Content="{x:Static lang:Resources.Common_Cancel}" Command="{Binding CancelCommand}"
+                    MinWidth="90" IsCancel="True"/>
+        </StackPanel>
+    </StackPanel>
+</dialogs:DialogWindow>