bread-theme: shared component stylesheet + generator CLI

Adds the single source of truth for bread GUI styling so the apps stop
each re-implementing (and drifting on) component CSS:

- stylesheet(&Palette): full component sheet (buttons, entries, switches,
  dropdowns, lists/rows/sidebars, cards, chips, scrollbars, headings) built
  from the design tokens + a canonical @define-color block (surface=color0,
  overlay=color7, accent=color4).
- render() / shared_css_path() / write_shared_css(): render for the current
  pywal palette and write to $XDG_RUNTIME_DIR/bread/theme.css.
- gtk::apply_shared(): load that file (or a rendered fallback) at APPLICATION
  priority and watch it, so every app recolours live with no rebuild.
- new `bread-theme` CLI (generate|path|print) — gtk-free, light. Run at
  session start and on palette change; apps pick it up via the file watch.

The contract is a CSS *file*, so apps stay decoupled from this crate's gtk4
version. Tests cover the stylesheet, path, and render helpers.

bread-theme/Cargo.toml

@@ -18,3 +18,9 @@ gtk4 = { version = "0.11", features = ["v4_12"], optional = true }
 # Enable GTK4 CSS provider helpers (breadbar, breadbox, breadpad use this).
 # bread (daemon) and breadcrumbs (CLI) depend on this crate without the feature.
 gtk = ["dep:gtk4"]
+
+# The generator CLI. It only touches the gtk-free lib API (render + write), so
+# it builds without the gtk feature and stays light.
+[[bin]]
+name = "bread-theme"
+path = "src/bin/bread-theme.rs"