helium/ui: add "Copy link" action to downloads context menu

[saberoueslati]

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Tested on (check one or more):

• Windows
• macOS
• Linux

---

I tested my PR only on Linux at the moment, leaving this as a draft until I test on Windows, I'm building helium there at this moment

Video of manual test below :

Screencast.from.2026-04-29.00-09-03.webm

Closes #897

Adds a "Copy link" option to the download context menu that copies the original download source URL to the clipboard. Available on in-progress, paused, finished, and interrupted downloads.

Changes

• New COPY_DOWNLOAD_LINK command wired through all model/controller layers
• Clipboard write implemented via ScopedClipboardWriter in DownloadUIModel::ExecuteCommand
• Menu item added to all four context menu states in download_ui_context_menu.cc
• Stats tracking added (kCopyDownloadLinkEnabled, kCopyDownloadLinkClicked)
• New IDS_DOWNLOAD_MENU_COPY_LINK string ("Copy link") in generated_resources.grd

[saberoueslati]

I had also errors in the CI/CD piepline, I'll fix them then make it ready for review

[nsk-47]

I had also errors in the CI/CD piepline, I'll fix them then make it ready for review

It is possible to have clear/clear from list option similar to firefox?