Web source: off-screen WebView2 capture + layout persistence + transparent bg

Bundle exception (2026-08-27): this lands prior-session work plus today's
transparency fix as one commit per creator instruction.

WebView2Manager (new): off-screen WebView2 control parked at -5000/-5000 in
the window's visual tree, captures via CapturePreviewAsync every 100 ms into
a BGRA8 VideoFrame. Source Width/Height sync to the control via
PropertyChanged. JS transparent-background injection runs on every
NavigationStarting/NavigationCompleted so the page's own CSS does not paint
white over the capture.

Transparency fix: DefaultBackgroundColor = System.Drawing.Color.Transparent
set on the WPF control before EnsureCoreWebView2Async. The WPF SDK doc says
the value is forwarded to the controller on init; the IDL (ICoreWebView2-
Controller2) confirms alpha=0 makes the capture alpha-preserving. The
prior-session 'CoreWebView2.BackgroundColor' path was the wrong property
name and has been replaced.

LayoutStore: WebUri column added (idempotent ALTER TABLE) and round-tripped
on save/load (schema v11).

MainViewModel: WebView2Manager constructed on Loaded; loaded web sources
re-registered; Source.WebUri PropertyChanged forwards to manager.

MainWindow: off-screen host Canvas added to visual tree.

Tests: 3 scale-sync + WebView2_DefaultBackgroundColor_Is_Transparent +
LayoutStorePersistenceTests.WebSource_WebUri_Persists. 234/236 pass
(2 pre-existing failures unchanged).

Scale bug (DPI/physical-pixel mismatch) still open.

Services/LayoutStore.cs

@@ -274,6 +274,13 @@ public class LayoutStore : IDisposable
             cmd.ExecuteNonQuery();
         }
 
+        if (!columns.Contains("WebUri"))
+        {
+            using var cmd = _connection.CreateCommand();
+            cmd.CommandText = "ALTER TABLE Source ADD COLUMN WebUri TEXT;";
+            cmd.ExecuteNonQuery();
+        }
+
         // Rename legacy "Backdrop" display name to "Background" in existing rows
         using (var cmd = _connection.CreateCommand())
         {
@@ -591,7 +598,8 @@ public class LayoutStore : IDisposable
                        X, Y, Width, Height, Opacity, MonitorIndex, ClipShape, IsMirrored,
                        IsBackground, CaptureKey, ShowDesktop,
                        ChatFontSize, ChatFontColor, ChatBackgroundColor, ChatMaxMessages,
-                       ChatShowTimestamps, ChatShowBadges, ChatMemberColor, ChatSuperChatColor
+                       ChatShowTimestamps, ChatShowBadges, ChatMemberColor, ChatSuperChatColor,
+                       WebUri
                 FROM Source ORDER BY SortOrder
                 """;
             using var reader = cmd.ExecuteReader();
@@ -628,6 +636,7 @@ public class LayoutStore : IDisposable
                     ChatShowBadges = reader.IsDBNull(22) ? true : reader.GetInt32(22) != 0,
                     ChatMemberColor = reader.IsDBNull(23) ? "#e94560" : reader.GetString(23),
                     ChatSuperChatColor = reader.IsDBNull(24) ? "#eab308" : reader.GetString(24),
+                    WebUri = reader.IsDBNull(25) ? null : reader.GetString(25),
                 };
                 if (!sourcesByScene.TryGetValue(sceneId, out var list))
                     sourcesByScene[sceneId] = list = new List<Source>();
@@ -837,12 +846,14 @@ public class LayoutStore : IDisposable
                                     X, Y, Width, Height, Opacity, MonitorIndex,
                                     ClipShape, IsMirrored, IsBackground, CaptureKey, ShowDesktop, SortOrder,
                                     ChatFontSize, ChatFontColor, ChatBackgroundColor, ChatMaxMessages,
-                                    ChatShowTimestamps, ChatShowBadges, ChatMemberColor, ChatSuperChatColor)
+                                    ChatShowTimestamps, ChatShowBadges, ChatMemberColor, ChatSuperChatColor,
+                                    WebUri)
                 VALUES ($id, $sceneId, $assetId, $type, $name, $isEnabled,
                         $x, $y, $w, $h, $opacity, $monitor,
                         $clip, $mirrored, $isBackground, $captureKey, $showDesktop, $sort,
                         $chatFontSize, $chatFontColor, $chatBgColor, $chatMaxMsgs,
-                        $chatTimestamps, $chatBadges, $chatMemberColor, $chatSuperChatColor)
+                        $chatTimestamps, $chatBadges, $chatMemberColor, $chatSuperChatColor,
+                        $webUri)
                 """;
             cmd.Transaction = tx;
             var idP = cmd.Parameters.Add("$id", SqliteType.Text);
@@ -871,6 +882,7 @@ public class LayoutStore : IDisposable
             var chatBadgesP = cmd.Parameters.Add("$chatBadges", SqliteType.Integer);
             var chatMemberColorP = cmd.Parameters.Add("$chatMemberColor", SqliteType.Text);
             var chatSuperChatColorP = cmd.Parameters.Add("$chatSuperChatColor", SqliteType.Text);
+            var webUriP = cmd.Parameters.Add("$webUri", SqliteType.Text);
 
             foreach (var scene in scenes)
             {
@@ -904,6 +916,7 @@ public class LayoutStore : IDisposable
                     chatBadgesP.Value = source.ChatShowBadges ? 1 : 0;
                     chatMemberColorP.Value = source.ChatMemberColor;
                     chatSuperChatColorP.Value = source.ChatSuperChatColor;
+                    webUriP.Value = (object?)source.WebUri ?? DBNull.Value;
                     cmd.ExecuteNonQuery();
                 }
             }