bread-theme: add libadwaita components + fix slider/chip theming gaps

New `adw` feature (gated separately from `gtk`, since AdwApplicationWindow
isn't compatible with gtk4-layer-shell — the five panel/launcher apps stay
on plain `gtk`, only breadman/breadhelp-style plain-window apps want this):
preferences_group/toggle_row/spin_row/action_row/preferences_page, wrapping
libadwaita's PreferencesGroup/SwitchRow/SpinRow/ActionRow/PreferencesPage.
adw::init() also forces dark color-scheme, since bread-theme's whole design
is a fixed dark base regardless of system GTK preference.

These directly target defects a design critique found: hand-rolled
switch+label rows with no intrinsic width (breadman/settings' ~1400px
stretched toggles) and spinners stranded far from their label — both just
don't happen when the row is a real AdwSwitchRow/AdwSpinRow instead of a
box assembled from scratch.

Also, two shared-stylesheet fixes usable by every app immediately, gtk
feature only:
- `scale` (slider) had no rule at all, so every volume/brightness slider
  showed GTK's own default blue instead of the palette accent — the same
  critique flagged breadbar's control-panel sliders contradicting its own
  on-brand OSD fill two clicks away.
- A new `chip()`/`set_chip_active()` helper in gtk.rs uses the existing
  (already-tokenized, already-defined) `.chip`/`.pill` stylesheet rule
  instead of each app hand-rolling its own filter-chip CSS — which is how
  breadclip/breadpad/breadman ended up with three different, mutually
  disagreeing pill fills for what's supposed to be one shared component.

bread-theme/src/gtk.rs

@@ -114,6 +114,29 @@ pub fn apply_css(css: &str, provider: &RefCell<Option<CssProvider>>) {
     }
 }
 
+/// A filter/tag chip using the shared `.chip` stylesheet rule (an
+/// `@overlay`-filled pill, `@accent`-filled when the `active` CSS class is
+/// set) instead of a fresh literal color — this is the fix for the same
+/// component drifting to three different fills across breadclip (grey),
+/// breadpad, and breadman (both cream), none of which agreed with each
+/// other or with the shared token.
+pub fn chip(label: &str) -> gtk4::Button {
+    gtk4::Button::builder().label(label).css_classes(["chip"]).build()
+}
+
+/// Toggles a chip's (or any widget's) `active` CSS class — the `.chip.active`
+/// stylesheet rule fills it with the accent instead of the neutral overlay.
+/// Wiring *when* a chip becomes active (single-select filter, multi-select
+/// tags, etc.) is genuinely per-app, so that stays the caller's job; this is
+/// just the one-line visual toggle every case needs.
+pub fn set_chip_active(chip: &impl IsA<gtk4::Widget>, active: bool) {
+    if active {
+        chip.add_css_class("active");
+    } else {
+        chip.remove_css_class("active");
+    }
+}
+
 /// Apply a user CSS override file at USER priority. Clears the provider if the
 /// file is absent so stale overrides don't persist across SIGHUP reloads.
 pub fn apply_user_css(path: &Path, provider: &RefCell<Option<CssProvider>>) {