Refactor & Extract image viewers (#1074)

* Extract `ImagePreviewView` + add `#Preview`

* Extract image pickers + dedupe and bg processing of images

* Include a dummy photo in the preview assets vs live url

* Fix development assets + url percent encoding

* Fix iOS vs macOS build issue

* Fix SPM-related issues

Package.swift

@@ -32,6 +32,7 @@ let package = Package(
             exclude: [
                 "Info.plist",
                 "Assets.xcassets",
+                "_PreviewHelpers/PreviewAssets.xcassets",
                 "bitchat.entitlements",
                 "bitchat-macOS.entitlements",
                 "LaunchScreen.storyboard",