v1.5.0-beta3

Build Linux AppImage for both x64 and arm64 (#60)

The prepare-whisper.mjs script compiles whisper.cpp natively for the
build host's architecture, so a single runner can't produce a correct
multi-arch AppImage. Split the Linux release job into per-arch matrix
entries (ubuntu-latest for x64, ubuntu-24.04-arm for arm64) so each
AppImage bundles whisper/ffmpeg binaries matching its own architecture.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>