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.
