fix: downloads from iframes/subframes now work (Gmail, embedded viewers) (#9)

[arzafran]

What this does

Clicking a download link inside an iframe (a Gmail attachment, an embedded PDF viewer's Download button, etc.) silently did nothing — the file never downloaded. The browser's response handler returned early for any subframe response before checking for a Content-Disposition: attachment header, so WebKit treated the download as inline navigation and dropped it. Downloads initiated from inside iframes now work.

Summary

In both BrowserNavigationDelegate (BrowserPanel.swift) and PopupNavigationDelegate (BrowserPopupWindowController.swift), the decidePolicyFor navigationResponse early-return for !isForMainFrame now first checks for Content-Disposition: attachment and returns .download when present, so the response reaches the existing BrowserDownloadDelegate. Non-attachment subframe responses still pass through as .allow.

Test Plan

• CI build passes
• Manual: open Gmail in a browser pane, download an attachment from inside the message iframe

Fixes #9