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/src/gtk.rs

@@ -1,7 +1,41 @@
+use gtk4::gio;
+use gtk4::prelude::*;
 use gtk4::CssProvider;
 use std::cell::RefCell;
 use std::path::Path;
 
+thread_local! {
+    static SHARED_PROVIDER: RefCell<Option<CssProvider>> = const { RefCell::new(None) };
+    static SHARED_MONITOR:  RefCell<Option<gio::FileMonitor>> = const { RefCell::new(None) };
+}
+
+fn reload_shared() {
+    let css = std::fs::read_to_string(crate::shared_css_path())
+        .unwrap_or_else(|_| crate::render());
+    SHARED_PROVIDER.with(|cell| apply_css(&css, cell));
+}
+
+/// Load the ecosystem's shared stylesheet (the file written by
+/// `bread-theme generate`, or a freshly rendered fallback if absent) at
+/// APPLICATION priority, and watch the file so the whole UI recolours live when
+/// the palette changes — no app rebuild or restart needed.
+///
+/// Call once at startup; then add the app's own CSS provider *after* this so
+/// app-specific rules win on equal specificity.
+pub fn apply_shared() {
+    reload_shared();
+    SHARED_MONITOR.with(|cell| {
+        if cell.borrow().is_some() {
+            return;
+        }
+        let file = gio::File::for_path(crate::shared_css_path());
+        if let Ok(monitor) = file.monitor_file(gio::FileMonitorFlags::NONE, gio::Cancellable::NONE) {
+            monitor.connect_changed(|_, _, _, _| reload_shared());
+            *cell.borrow_mut() = Some(monitor);
+        }
+    });
+}
+
 /// Apply a CSS string to the default display at APPLICATION priority.
 /// Re-uses an existing provider if one is passed in (for SIGHUP reloads).
 pub fn apply_css(css: &str, provider: &RefCell<Option<CssProvider>>) {