fix(ui): refine command line and clipboard

- preserve command history colors after reopening
- remove collapsed command bar residue and split the unsaved prompt
- read system text when the drawing clipboard is empty

Refs #467
Refs #468
Refs #470

src/app/mod.rs

@@ -1586,10 +1586,11 @@ pub enum Message {
         crate::scene::text::web_font::Script,
         Result<(), String>,
     ),
-    /// Ctrl+V. Routed by `update`: into the open text/MText editor (via an async
-    /// system-clipboard read, which is the only paste path that works on the
-    /// web) or, with no editor open, the entity paste command.
+    /// Ctrl+V. Routed by `update` into an open text editor, the drawing-object
+    /// clipboard, or the system text clipboard.
     PasteShortcut,
+    /// Completion of a system text clipboard read requested by PASTECLIP.
+    SystemClipboardPaste(SystemClipboardText),
     /// Ctrl/Cmd+A — select all layer rows when the Layer Manager is open, or all
     /// drawing objects otherwise (#236).
     SelectAllShortcut,
@@ -2791,6 +2792,15 @@ pub enum Message {
     ),
 }
 
+#[derive(Debug, Clone)]
+pub enum SystemClipboardText {
+    Text(String),
+    EmptyOrUnsupported,
+    Unavailable,
+    Occupied,
+    ConversionFailed,
+}
+
 impl OpenCADStudio {
     /// Install the Start-page video list, decoding each thumbnail's JPEG into
     /// an image Handle exactly once (a fresh Handle per view frame would