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>

Cargo.toml

@@ -136,6 +136,13 @@ web-sys = { version = "0.3", features = [
     # Async clipboard read for paste into the text/MText editors (iced's own
     # clipboard read is a no-op on the web).
     "Clipboard",
+    # Web copy/paste in plain text_input fields (#346): Ctrl+C writes the
+    # focused field's text; Ctrl+V replays the clipboard as synthetic
+    # KeyboardEvents on the canvas (the only route into a focused text_input).
+    "KeyboardEvent",
+    "KeyboardEventInit",
+    "EventInit",
+    "EventTarget",
     # Error banner (web_diag): forward wgpu/naga errors to the browser console
     # and inject a copyable DOM overlay (document.body → append_child).
     "console",