Drop pacman packaging, bakery-only; fix broken Tauri build path in CI

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.

bakery.toml

@@ -1,9 +1,11 @@
 name = "bos-settings"
 description = "System settings app for Bread OS"
 binaries = ["bos-settings"]
-system_deps = ["gtk4", "glib2"]
+system_deps = ["gtk4", "glib2", "hicolor-icon-theme"]
 optional_system_deps = ["snapper"]
 bread_deps = []
+license_file = "LICENSE"
+desktop_file = "bos-settings.desktop"
 
 [config]
 dir = "~/.config"