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

## 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 ```swift 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: ```swift 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.