v1.6.0-Beta2

Mobile-focused release: download fixes, plus a search order change.

• Fix: Anna's Archive on Android: the DDoS check actually cleared but the app gave up a moment too early on some devices; it now waits for the page properly. When the invisible check still fails, a visible browser window opens instead, closes itself when done.
• New setting: "Verify challenges in background": headless solving works on most devices but not all. If verification keeps failing, turn this off and the app will show the visible window instead. The blocked-content error page now points at this toggle too.
• Search order: LibGen first: LibGen serves direct downloads without any browser challenge, so it's now the default and gets tried first. If it comes up empty, Z-Library runs, and Anna's Archive only after that. Existing installs keep their current provider selection.
• Fix: Z-Library book pages: opening a Z-Library result used to route through Anna's Archive (which doesn't have the book) and died with a 404. Book pages and downloads now go through Z-Library itself, including its proof-of-work wall and the CDN redirect.
• Fix: per-ABI Android builds are back in the release pipeline.