Media: Add HEIC canvas fallback for client-side processing

[adamsilverstein]

Description

HEIC (HEIF with HEVC codec) images are the default photo format on iPhones but are not universally supported by web servers or browsers. Currently, if the server lacks HEIC support (no ImageMagick HEVC module), uploads may fail entirely because wp_prevent_unsupported_mime_type_uploads blocks them.

Proposed Solution

Implement a server-first, canvas-fallback strategy:

1. Upload HEIC to server, letting it try to process
2. Sideload the original HEIC to the attachment as image_size: 'original' — the sideload response returns missing_image_sizes so we know what sub-sizes need to be generated
3. If there are missing sizes, use browser-native createImageBitmap() + OffscreenCanvas to decode the HEIC and convert to JPEG
4. Sideload the JPEG as the scaled version (web-friendly main image)
5. Use the JPEG with the existing vips resize pipeline for sub-size generation
6. Sideload each sub-size back to the attachment

This leverages browser-licensed HEVC decoding (Safari everywhere, Chrome on macOS, Chrome on Windows with codec) without shipping our own HEVC decoder, avoiding patent/licensing concerns.

Scenarios

Server HEIC support	Browser HEIC support	Result
Yes	Any	Server handles everything
No	Yes	Canvas fallback generates JPEG + sub-sizes
No	No	Error shown to user

Implementation

PR: #76731