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 <[email protected]>

src/app/mod.rs

@@ -1265,6 +1265,17 @@ pub enum Message {
     OpenPathPicked(Option<(PathBuf, u64)>),
     /// A file was dragged from the desktop and dropped on the window (#344).
     FileDropped(PathBuf),
+    /// Web only: Ctrl+V captured by a focused text field, whose iced-internal
+    /// paste is inert there — read the async clipboard instead (#346).
+    WebFieldPaste,
+    /// Web only: clipboard text arrived for a focused field — replay it as
+    /// synthetic keystrokes on the canvas.
+    WebFieldPasteText(Option<String>),
+    /// Web only: Ctrl+C captured by a focused text field — collect the
+    /// focused input's text via a widget operation.
+    WebFieldCopy,
+    /// Web only: the focused field's text — write it to the clipboard.
+    WebFieldCopyText(Option<String>),
     /// Pick from the one-shot snap override menu (Shift+RMB): only this snap
     /// applies to the next point pick, then the configuration restores (#337).
     SnapOverridePick(crate::snap::SnapType),