PostView.handle_upload blocks main actor with synchronous file I/O

[alltheseas]

Problem

PostView.handle_upload(media:) calls getImage(media:) synchronously on the main actor before any await. This does file I/O (Data(contentsOf:)) and potentially video thumbnail generation on the main thread, which can cause UI hitches — especially with large media files.

Location: damus/Features/Posting/Views/PostView.swift around line 465-478

func handle_upload(media: MediaUpload) async -> Bool {
    mediaUploadUnderProgress = media
    let uploader = damus_state.settings.default_media_uploader
    
    let img = getImage(media: media)  // ← synchronous file I/O on main actor
    print("img size w:\(img.size.width) h:\(img.size.height)")
    
    async let blurhash = calculate_blurhash(img: img)
    // ...
}

Suggested fix

Move getImage(media:) and calculate_blurhash(img:) to a background context, then hop back to @MainActor for UI state updates and upload dispatch:

func handle_upload(media: MediaUpload) async -> Bool {
    mediaUploadUnderProgress = media
    let uploader = damus_state.settings.default_media_uploader

    let (img, blurhash) = await Task.detached {
        let img = getImage(media: media)
        let blurhash = calculate_blurhash(img: img)
        return (img, blurhash)
    }.value

    // Continue on MainActor with upload...
}

Impact

Low-to-medium — only manifests with large images/videos where the synchronous read is noticeable. Won't crash, but can cause frame drops during upload initiation.

[linear]

IOS-2356