[teidesu]

an attempt to make scrolling the chat history smoother on macos. two parts: render scroll positions with sub-pixel precision (needs desktop-app/lib_ui#324), and stop decoding images on the main thread, which caused visible stutter while scrolling media-heavy chats. it's not perfect but noticeably smoother.

• skip thumbnail bytes generation for local stickers — local stickers were re-encoding a WEBP thumbnail that was never used; skip it via a new generateThumbnailBytes flag.
• decode sticker images without blocking the main thread — checkStickerLarge constructed the Image (synchronous decode) on the main thread; move the decode to crl::async with a loading guard.
• decode downloaded and partial cached images asynchronously — FileLoader now also decodes partial:-prefixed cache entries off-thread when the partial data covers the requested size, and decodes freshly downloaded images on crl::async before firing updates instead of leaving it to the consumer on the main thread.
• apply sub-pixel scroll offset in history view — HistoryInner subscribes to the scroll view's sub-pixel offset, translates painting by the fraction (keeping the floating date badge and forum bar pinned) and compensates in all hit-testing paths.

note: the code in this pr was written partially by claude fable 5 and partially by gpt-5.6-sol.

[Prana111025]

Changed

[teidesu]

removed subpixel scrolling related changes as per desktop-app/lib_ui#324 (comment)

keeping the async decoding fixes still