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/lib.rs

@@ -1,6 +1,8 @@
 pub mod palette;
 #[cfg(feature = "gtk")]
 pub mod gtk;
+#[cfg(feature = "adw")]
+pub mod adw;
 
 pub use palette::{load_palette, Palette};
 
@@ -187,6 +189,14 @@ pub fn stylesheet(p: &Palette) -> String {
          switch {{ background-color: @overlay; border-radius: {pill}px; }}\n\
          switch:checked {{ background-color: @accent; }}\n\
          switch slider {{ background-color: @on-surface; border-radius: {pill}px; }}\n\
+         /* GtkScale (sliders) render with GTK's own default accent (a fixed\
+            blue, independent of the app's theme) unless styled explicitly —\
+            every app with a volume/brightness slider was silently showing\
+            that default instead of the palette's accent until this rule\
+            existed. */\n\
+         scale trough {{ background-color: @overlay; border-radius: {pill}px; min-height: 6px; }}\n\
+         scale trough highlight {{ background-color: @accent; border-radius: {pill}px; min-height: 6px; }}\n\
+         scale slider {{ background-color: @on-bg; border-radius: {pill}px; }}\n\
          list, listbox {{ background-color: transparent; }}\n\
          row {{ border-radius: {r2}px; }}\n\
          row:selected, list row:selected {{ background-color: @accent; color: @on-accent; }}\n\