Drive bar/window geometry and CSS tokens from the shell theme manifest

breadbar no longer hardcodes BAR_HEIGHT/BAR_MARGIN_TOP/BAR_MARGIN_SIDES/
CHIP_HEIGHT/ICON_PX or the root window's layer-shell setup: it loads
bread_theme:🐚:ShellTheme once (theme::shell_theme(), cached in a
thread-local) and reads bar.window (anchors, margin, exclusive zone,
keyboard mode - now set explicitly instead of relying on the library
default) plus tokens.{icon_px,chip_height} from it everywhere those used
to be literals.

The four satellite surfaces (breadbar-osd, breadbar-notif, breadbar-panel,
breadbar-dismiss) now get their anchor/margin/width/layer from the
manifest's [surfaces.*] table via a small new surface::apply() helper,
narrowly scoped to the three anchor shapes those surfaces actually use.
The pre-existing 8px gap between the panel's top margin and the dismiss
scrim's is preserved exactly (and now commented) rather than "fixed".

theme.rs's load_css() keeps its ~250 lines of hand-written breadbar CSS
(notifications, wifi popover, control panel, media widget) but now reads
its five radius/pad locals and the two easing curves (spring vs
spring_settle - hover/settle transitions were previously miscategorized
as the overshoot curve in the constant audit; the actual code already
used the settle curve there, confirmed against src) from theme tokens
instead of hardcoding them.

Also wires bread_theme:🐚:watch() so editing the active theme's
theme.toml/extra.css hot-reloads CSS tokens without a restart, same as
a pywal palette change already does. Window-spec values still need a
restart per the plan (read once at window-construction time).

Verified pixel-identical: captured all 10 breadbar --screenshot views
(bar, control-panel, connectivity-wifi/bluetooth, media-popover,
notification/-critical, osd-volume/-brightness, wifi-add-dialog) via
bread-capture's isolated headless-Sway harness against both the
pre-change and post-change binaries; every view diffs byte-identical
at the decoded-pixel level.

src/bar/workspaces.rs

@@ -142,7 +142,7 @@ pub fn make_button(
     btn.set_halign(gtk4::Align::Center);
     btn.set_vexpand(false);
     btn.set_hexpand(false);
-    btn.set_size_request(-1, crate::CHIP_HEIGHT);
+    btn.set_size_request(-1, crate::theme::shell_theme().tokens().chip_height() as i32);
     if let Some(child) = btn.child() {
         child.set_halign(gtk4::Align::Center);
         child.set_valign(gtk4::Align::Center);