feat(web): copy/paste in text fields via the browser clipboard (#346)

iced's clipboard rides window_clipboard, which has no wasm backend — a
focused text_input captured Ctrl+C/V on the web and did nothing.

Ctrl+V (captured, wasm only): read the async browser clipboard (the
keypress is a user gesture, so it's permitted) and replay the text as
synthetic KeyboardEvents on the winit canvas — the only route into a
focused text_input, since iced exposes no insert-text operation.
Control characters are dropped and the length is capped. The MText and
inline-TEXT editors keep their dedicated paste paths (no double feed),
and an unfocused Ctrl+V still runs PASTECLIP.

Ctrl+C (captured, wasm only): a widget operation pairs each text_input
with its focus state (iced fires both callbacks back-to-back on the
same widget) to collect the focused field's visible text, then writes
it with navigator.clipboard.writeText. Whole-field text, not the
selection — iced doesn't expose the selected range.

Native behaviour is unchanged.

Closes #346

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/view/mod.rs

@@ -1901,6 +1901,13 @@ impl OpenCADStudio {
                                     Some(Message::Command("CUTCLIP".to_string()))
                                 }
                                 "v" if status == Status::Ignored => Some(Message::PasteShortcut),
+                                // Web: a focused text field captured Ctrl+C/V,
+                                // but iced's clipboard is a no-op there — route
+                                // through the async browser clipboard (#346).
+                                #[cfg(target_arch = "wasm32")]
+                                "v" => Some(Message::WebFieldPaste),
+                                #[cfg(target_arch = "wasm32")]
+                                "c" => Some(Message::WebFieldCopy),
                                 _ => None,
                             },
                             // Printable glyphs are already handled by the