Replaces the dev/beta branch split with one trunk (main): dev-track
builds still publish on every push, but the beta track now publishes
from a vX.Y.Z-rc.N prerelease tag instead of a separately-maintained
beta branch. Removes the branch nobody reliably kept in sync.
bakery.toml gains license_file/desktop_file (closing the same gap found
across the rest of the ecosystem) plus hicolor-icon-theme as an optional
dep. Removes packaging/PKGBUILD and .forgejo/workflows/package.yml.
Also fixes release.yml, which has never actually succeeded since the
GTK4->Tauri migration (93c3b88): Cargo.toml moved to src/src/Cargo.toml
(the checkout dir is conventionally named "src", and this repo's own
Rust backend directory is *also* named "src", not the usual src-tauri),
and the Rust build needs frontend/build to already exist (tauri.conf.json's
frontendDist) — that's normally handled by `cargo tauri build`'s
beforeBuildCommand hook, which a plain `cargo build` never runs, so it's
now a separate explicit "build frontend" step. Adds dev-release.yml/
beta-release.yml for the same three-track policy used ecosystem-wide.
