Added block channel feature - #984

Full disclosure: This entire change has been generated using AI, build tested and verified working on Windows 11 x64 25H2

I assume the big red BLOCK button on channels and videos can use some adjustment.

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Adds the ability to block channels across the app.

What's included

• Block a channel from the home screen context menu, the video detail screen, and the channel page.
• New Blocked Channels page (sidebar -> Blocked channels) listing blocked channels with unblock support.
• Content filtering: blocked channels are filtered out of the home feed and subscriptions via the new FilterPager<T> (see Engine PR).
• Playback enforcement: video sources, HLS/DASH, downloads, and playback attempts for blocked channels return a clear "Channel blocked" dialog.
• Sync support: blocked channels (and unblock tombstones) sync between devices via SyncBlockedChannels.
• Backup/restore: blocked channels are included in backups and imports.
• Tests: BlockedChannelsTests + FilterPagerTests (24 tests).

Dependencies

Depends on futo-org/Grayjay.Engine#2 — this branch pins the Engine at the commit adding FilterPager<T> and the RefreshDistributionContentPager<T> filter overload. The submodule pointer should be updated to the upstream Engine commit once that PR merges.

Note: includes rebuilt frontend assets (font/asset filename churn) from npm run build.