fix(web): paste clipboard text into the TEXT / MText editors

Ctrl+V was globally bound to the entity paste command, so it never pasted
into an open text editor, and even when routed there iced's clipboard read
is a no-op on the web. Route Ctrl+V to the focused editor and, on the web,
read the system clipboard via the browser's async Clipboard API (the
keypress is a user gesture, so the read is permitted):

- MText editor: insert at the caret (web + native).
- TEXT field: append on the web; native lets the text_input paste itself.
- No editor open: falls through to the entity paste command as before.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/view.rs

@@ -1769,7 +1769,7 @@ impl OpenCADStudio {
                                 "y" => Some(Message::Redo),
                                 "c" => Some(Message::Command("COPYCLIP".to_string())),
                                 "x" => Some(Message::Command("CUTCLIP".to_string())),
-                                "v" => Some(Message::Command("PASTECLIP".to_string())),
+                                "v" => Some(Message::PasteShortcut),
                                 _ => None,
                             },
                             // Printable glyphs are already handled by the