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>

Cargo.toml

@@ -106,6 +106,9 @@ web-sys = { version = "0.3", features = [
     "Blob",
     "Url",
     "Response",
+    # Async clipboard read for paste into the text/MText editors (iced's own
+    # clipboard read is a no-op on the web).
+    "Clipboard",
 ] }
 # Browsers without WebGPU (e.g. Firefox by default) need wgpu's WebGL2 backend;
 # `webgl` enables it. `fira-sans` embeds the default UI font — the web has no