Add HEIC image support on Linux via flutter_image_compress_linux

[hmcqueen]

Adds HEIC image rendering support on Linux desktop.

Problem

HEIC images sent from iPhones display "Failed to display image" on Linux (#117). This is because flutter_image_compress has no Linux platform implementation, and the HEIC conversion code in attachments_service.dart was gated behind !kIsDesktop, skipping it entirely on desktop platforms.

Solution

• Added flutter_image_compress_linux as a dependency — a federated platform implementation that provides JPEG, PNG, WebP, and HEIC compression on Linux via dart:ffi with native C codecs (libjpeg-turbo, libpng, libwebp, libheif)
• Changed !kIsDesktop to !Platform.isWindows so HEIC conversion runs on Linux and macOS (which already has flutter_image_compress_macos), while still skipping Windows which has no implementation yet

Changes

• pubspec.yaml: Added flutter_image_compress_linux: ^0.1.0 to dependencies
• lib/services/ui/attachments_service.dart: Changed !kIsDesktop to !Platform.isWindows on line 349

Testing

• Built and tested locally on Pop!_OS 24.04
• Confirmed HEIC images from iPhone conversations render correctly
• All existing image paths (attachment picker, message bubbles, fullscreen viewer, media gallery) flow through the same loadAndGetProperties() method, so the single change covers all cases

Addresses #117 (Linux). Windows HEIC support would require a separate flutter_image_compress_windows package.