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/Cargo.toml

@@ -13,11 +13,24 @@ serde = { workspace = true }
 serde_json = { workspace = true }
 dirs = { workspace = true }
 gtk4 = { version = "0.11", features = ["v4_12"], optional = true }
+# Rust bindings for libadwaita (GNOME's widget library on top of GTK4) — the
+# actual source of the modern GNOME look (grouped preference rows, real
+# toggle/spin rows, view switchers), not just a CSS reskin of plain GTK4
+# widgets. `v1_7` for ToggleGroup (used for tab-row-style pickers); the
+# system library only needs to be >= that (this machine has 1.9.2).
+libadwaita = { version = "0.9", features = ["v1_7"], optional = true }
 
 [features]
 # Enable GTK4 CSS provider helpers (breadbar, breadbox, breadpad use this).
 # bread (daemon) and breadcrumbs (CLI) depend on this crate without the feature.
 gtk = ["dep:gtk4"]
+# Composite libadwaita-based widgets (bread_theme::adw) — separate from `gtk`
+# because libadwaita's own top-level window chrome (AdwApplicationWindow)
+# isn't compatible with gtk4-layer-shell surfaces, so the five layer-shell
+# apps (breadbar, breadbox, breadclip, breadsearch, breadpad) only want
+# plain CSS, not this. Apps with an ordinary top-level window (breadman,
+# breadhelp) want both.
+adw = ["gtk", "dep:libadwaita"]
 
 # The generator CLI. It only touches the gtk-free lib API (render + write), so
 # it builds without the gtk feature and stays light.