feature: Add Clipboard API support to uni-dom (#404)

## Summary
- Add `Clipboard` object for reading and writing to the system clipboard
- Provides both async (Future-based) and reactive (Rx-based) APIs
- Includes event handlers for copy/cut/paste interception
- Falls back to `execCommand` for older browsers

## Features

```scala
import wvlet.uni.dom.all.*

// Write text to clipboard
Clipboard.writeText("Hello, World!").foreach(_ => println("Copied!"))

// Read text from clipboard  
Clipboard.readText().foreach(text => println(s"Pasted: ${text}"))

// One-click copy button
button(
  Clipboard.copyOnClick("Copy this text", onSuccess = () => showToast("Copied!")),
  "Copy"
)

// Dynamic copy (evaluates at click time)
button(
  Clipboard.copyOnClickDynamic(() => inputRef.current.map(_.value).getOrElse("")),
  "Copy Input"
)

// Intercept paste events
input(
  Clipboard.onPaste { text =>
    println(s"User pasted: ${text}")
  }
)

// Customize what gets copied
div(
  Clipboard.copyAs(() => "Custom formatted text"),
  "Select and copy me"
)

// Check browser support
if Clipboard.isSupported then
  // Use modern API
else
  // Fallback behavior
```

## API

| Method | Description |
|--------|-------------|
| `writeText(text)` | Write text to clipboard (Future) |
| `readText()` | Read text from clipboard (Future) |
| `writeTextRx(text)` | Write text, returns Rx[Boolean] |
| `onCopy(handler)` | Handle copy events |
| `onCut(handler)` | Handle cut events |
| `onPaste(handler)` | Handle paste events |
| `copyAs(getText)` | Customize copied content |
| `cutAs(getText)` | Customize cut content |
| `copyOnClick(text)` | Copy on click |
| `copyOnClickDynamic(getText)` | Copy dynamic text on click |
| `isSupported` | Check if Clipboard API is available |

## Test plan
- [x] All 257 tests pass (17 new clipboard tests)
- [x] Compilation successful
- [x] Code formatted with scalafmtAll

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.5 <[email protected]>

Author: xerial

uni-dom-test/src/test/scala/wvlet/uni/dom/ClipboardTest.scala

@@ -0,0 +1,130 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package wvlet.uni.dom
+
+import org.scalajs.dom
+import wvlet.uni.test.UniTest
+import wvlet.uni.dom.all.*
+import wvlet.uni.dom.all.given
+import wvlet.uni.rx.Rx
+
+import scala.concurrent.Future
+
+class ClipboardTest extends UniTest:
+
+  test("Clipboard.isSupported returns Boolean"):
+    val supported = Clipboard.isSupported
+    supported shouldMatch { case _: Boolean =>
+    }
+
+  test("Clipboard.writeText returns Future[Unit]"):
+    val result = Clipboard.writeText("test")
+    result shouldMatch { case _: Future[?] =>
+    }
+
+  test("Clipboard.readText returns Future[String]"):
+    val result = Clipboard.readText()
+    result shouldMatch { case _: Future[?] =>
+    }
+
+  test("Clipboard.writeTextRx returns Rx[Option[Boolean]]"):
+    val result = Clipboard.writeTextRx("test")
+    result shouldMatch { case _: Rx[?] =>
+    }
+
+  test("Clipboard.onCopy returns DomNode"):
+    val node = Clipboard.onCopy(_ => ())
+    node shouldMatch { case _: DomNode =>
+    }
+
+  test("Clipboard.onCut returns DomNode"):
+    val node = Clipboard.onCut(_ => ())
+    node shouldMatch { case _: DomNode =>
+    }
+
+  test("Clipboard.onPaste returns DomNode"):
+    val node = Clipboard.onPaste(_ => ())
+    node shouldMatch { case _: DomNode =>
+    }
+
+  test("Clipboard.onPasteEvent returns DomNode"):
+    val node = Clipboard.onPasteEvent(_ => ())
+    node shouldMatch { case _: DomNode =>
+    }
+
+  test("Clipboard.copyAs returns DomNode"):
+    val node = Clipboard.copyAs(() => "text")
+    node shouldMatch { case _: DomNode =>
+    }
+
+  test("Clipboard.cutAs returns DomNode"):
+    val node = Clipboard.cutAs(() => "text")
+    node shouldMatch { case _: DomNode =>
+    }
+
+  test("Clipboard.copyOnClick returns DomNode"):
+    val node = Clipboard.copyOnClick("text")
+    node shouldMatch { case _: DomNode =>
+    }
+
+  test("Clipboard.copyOnClick with callbacks returns DomNode"):
+    var success = false
+    val node = Clipboard.copyOnClick("text", onSuccess = () => success = true, onFailure = _ => ())
+    node shouldMatch { case _: DomNode =>
+    }
+
+  test("Clipboard.copyOnClickDynamic returns DomNode"):
+    val node = Clipboard.copyOnClickDynamic(() => "dynamic text")
+    node shouldMatch { case _: DomNode =>
+    }
+
+  test("Clipboard.copyOnClickDynamic with callbacks returns DomNode"):
+    var success = false
+    val node    = Clipboard.copyOnClickDynamic(
+      () => "dynamic text",
+      onSuccess = () => success = true,
+      onFailure = _ => ()
+    )
+    node shouldMatch { case _: DomNode =>
+    }
+
+  test("Clipboard handlers can be attached to elements"):
+    val elem = div(
+      Clipboard.onCopy(text => ()),
+      Clipboard.onCut(text => ()),
+      Clipboard.onPaste(text => ()),
+      input(placeholder -> "Test input")
+    )
+    elem shouldMatch { case _: RxElement =>
+    }
+
+  test("Clipboard.copyOnClick can be attached to button"):
+    val elem = button(Clipboard.copyOnClick("Hello, World!"), "Copy")
+    elem shouldMatch { case _: RxElement =>
+    }
+
+  test("Clipboard event handlers render correctly"):
+    val elem           = div(Clipboard.onPaste(_ => ()))
+    val (node, cancel) = DomRenderer.createNode(elem)
+
+    node match
+      case e: dom.Element =>
+        // The element should exist
+        e.tagName.toLowerCase shouldBe "div"
+      case _ =>
+        fail("Expected Element")
+
+    cancel.cancel
+
+end ClipboardTest