Attach images pasted into the composer, comments and chat

Pressing Ctrl+V with a screenshot on the clipboard now attaches it
immediately instead of requiring a trip through the file picker.

The clipboard reader lives in dp-upload behind a new opt-in `paste`
boolean attribute: with it set, the component binds one paste listener
on its closest form and routes the clipboard image files through the
same handleFiles path as the picker and the drop target, so validation,
limits, the terms gate and the hidden attachment_uids field are shared.
A paste carrying plain text is never swallowed.

It is opt-in rather than a form-wide default because a form may hold
several upload buttons - projects.html has cover and logo beside the
attachment one - and a default would attach one pasted screenshot to
all of them.

Set on _attachment_form.html, so every form including it inherits the
behaviour (post composer, post edit, gists, projects, issues,
screenshots), plus _comment_form.html, messages.html and the embed-mode
skeleton AppChat builds.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/conftest.py

@@ -319,6 +319,31 @@ def login_user(page, user):
     page.wait_for_url("**/feed", timeout=10000, wait_until="domcontentloaded")
 
 
+def paste_image(page, selector, name="pasted.png"):
+    import base64
+    import io
+    from PIL import Image
+
+    buf = io.BytesIO()
+    Image.new("RGB", (4, 4), (0, 128, 255)).save(buf, "PNG")
+    page.eval_on_selector(
+        selector,
+        """(target, [data, filename]) => {
+            const bytes = Uint8Array.from(atob(data), (ch) => ch.charCodeAt(0));
+            const transfer = new DataTransfer();
+            transfer.items.add(new File([bytes], filename, { type: "image/png" }));
+            target.dispatchEvent(
+                new ClipboardEvent("paste", {
+                    clipboardData: transfer,
+                    bubbles: true,
+                    cancelable: true,
+                })
+            );
+        }""",
+        [base64.b64encode(buf.getvalue()).decode(), name],
+    )
+
+
 def assert_share_copies(page, expected_fragment):
     from playwright.sync_api import expect