Refactor theme onto bread-theme; add bakery.toml and release workflow

- breadpad-shared/Cargo.toml: depend on bread-theme (no gtk feature needed
  in the shared crate)
- breadpad-shared/src/theme.rs: re-export Palette and load_palette from
  bread-theme; retain all breadpad-specific CSS in build_css()
- bakery.toml: describes breadpad for bakery install
- release.yml: builds on hestia self-hosted runner, publishes binaries to
  dl.breadway.dev and GitHub Releases on v* tags

breadpad-shared/Cargo.toml

@@ -7,6 +7,9 @@ authors.workspace = true
 
 
 [dependencies]
+# Path dep for local dev; replace with git dep on first tag:
+# bread-theme = { git = "https://github.com/Breadway/bread-ecosystem", tag = "theme-v0.1.0" }
+bread-theme = { path = "../../bread-ecosystem/bread-theme" }
 anyhow.workspace = true
 tracing.workspace = true
 serde.workspace = true