fix(ui): report failed paste/drop in the interactive terminal

PasteAsync silently swallowed every exception and returned void, so a
package update renaming Iciclecreek's private write seam would break
Ctrl+V/drop with zero user-visible signal. PasteAsync/SendTextAsync now
return bool (mirroring SendLineAsync), and the view checks the result,
routing a failure through InteractiveTerminalViewModel.ErrorReported ->
ConPtyPaneViewModel.ErrorReported -> the existing footer error strip.

tests/ClaudeDo.Ui.Tests/Services/PtyTerminalSessionTests.cs

@@ -35,4 +35,14 @@ public class PtyTerminalSessionTests
 
         Assert.Equal(0, gate.CurrentCount); // acquired
     }
+
+    // Fix: PasteAsync used to be void and swallow every failure. Not-started is the cheap case to
+    // cover without a real TerminalControl/ConPTY spawn.
+    [Fact]
+    public async Task PasteAsync_ReturnsFalse_WhenNotStarted()
+    {
+        using var session = new PtyTerminalSession();
+
+        Assert.False(await session.PasteAsync());
+    }
 }