feat(spv): verify Merkle proofs progressively, gate spendability on it

Balance/history now render as soon as tx downloads finish instead of
blocking on every historical Merkle proof, critical for mobile where
proof-checking can take much longer than the download itself. Proofs
continue to be checked in the background and each tx's Verified flag
catches up progressively; header ranges are now fetched in batches
(blockchain.block.headers) instead of one call per header to keep this
fast over high-latency links.

Coin selection (UtxoSpendability.IsSpendable) refuses to spend a UTXO
until its Merkle proof is actually checked, regardless of confirmation
count, so a server that fabricates a confirmed balance can get it
displayed early but never spent before the forgery is caught. The disk
cache only ever persists the fully-verified end state of a sync.

UI surfaces the new PendingVerificationSats/SpendableSats split with a
"verifying..." badge, and the sync save now runs off the UI thread to
avoid freezing on slower hardware.

src/App/Views/MainView.axaml

@@ -317,6 +317,9 @@
                     <TextBlock Text="{Binding ImmatureText}" Foreground="#FCD34D"
                                TextWrapping="Wrap"
                                IsVisible="{Binding ImmatureText, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+                    <TextBlock Text="{Binding VerifyingText}" Foreground="#FCD34D"
+                               TextWrapping="Wrap"
+                               IsVisible="{Binding VerifyingText, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
                     <TextBlock Text="{Binding NetworkInfo}" Classes="on-hero" FontSize="12"
                                Margin="0,2,0,0"/>
                 </StackPanel>
@@ -373,13 +376,16 @@
                                 <DataTemplate x:DataType="vm:HistoryRow">
                                     <Panel Cursor="Hand">
                                         <!-- Desktop: 3 fixed columns -->
-                                        <Grid ColumnDefinitions="90,160,*"
+                                        <Grid ColumnDefinitions="90,160,*,Auto"
                                               IsVisible="{Binding $parent[UserControl].((vm:MainWindowViewModel)DataContext).IsDesktop}">
                                             <TextBlock Grid.Column="0" Text="{Binding Conferma}" Foreground="{DynamicResource TextSecondaryBrush}"/>
                                             <TextBlock Grid.Column="1" Text="{Binding Importo}" FontFamily="monospace"/>
                                             <TextBlock Grid.Column="2" Text="{Binding Txid}"
                                                        FontFamily="monospace" FontSize="12"
                                                        TextTrimming="CharacterEllipsis"/>
+                                            <TextBlock Grid.Column="3" Text="{Binding $parent[UserControl].((vm:MainWindowViewModel)DataContext).Loc[history.unverified]}"
+                                                       Foreground="#FCD34D" FontSize="11" Margin="6,0,0,0"
+                                                       IsVisible="{Binding !Verified}"/>
                                         </Grid>
                                         <!-- Mobile: vertical card -->
                                         <StackPanel Spacing="2"
@@ -389,6 +395,9 @@
                                             <TextBlock Text="{Binding Conferma}" Foreground="{DynamicResource TextSecondaryBrush}" FontSize="11"/>
                                             <TextBlock Text="{Binding Txid}" FontFamily="monospace" FontSize="11"
                                                        TextTrimming="CharacterEllipsis"/>
+                                            <TextBlock Text="{Binding $parent[UserControl].((vm:MainWindowViewModel)DataContext).Loc[history.unverified]}"
+                                                       Foreground="#FCD34D" FontSize="11"
+                                                       IsVisible="{Binding !Verified}"/>
                                         </StackPanel>
                                     </Panel>
                                 </DataTemplate>