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.

Cargo.lock

@@ -199,6 +199,7 @@ version = "0.3.1"
 dependencies = [
  "dirs",
  "gtk4",
+ "libadwaita",
  "serde",
  "serde_json",
 ]
@@ -1279,6 +1280,36 @@ version = "3.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e2db585e1d738fc771bf08a151420d3ed193d9d895a36df7f6f8a9456b911ddc"
 
+[[package]]
+name = "libadwaita"
+version = "0.9.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "85b9900e67182a4b5b1f157b448d94f0715c8b9770cce21cf000801917f53bfa"
+dependencies = [
+ "gdk4",
+ "gio",
+ "glib",
+ "gtk4",
+ "libadwaita-sys",
+ "pango",
+]
+
+[[package]]
+name = "libadwaita-sys"
+version = "0.9.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "28d3c27642b389852aa99341bd4a4c19ec6f8a2b63ebdd7f5ba1952198079ccd"
+dependencies = [
+ "gdk4-sys",
+ "gio-sys",
+ "glib-sys",
+ "gobject-sys",
+ "gtk4-sys",
+ "libc",
+ "pango-sys",
+ "system-deps",
+]
+
 [[package]]
 name = "libc"
 version = "0.2.186"