refactor(slint): extract toast into qbz-slint-common crate

New shared crate qbz-slint-common (will also host the artwork primitives via a
dependency-inversion pass). toast.rs is a clean leaf over qbz-ui: only Slint
types (AppWindow, ToastKind, ToastState) + slint core (Timer/Weak). Moved
verbatim; the only edit is the import (`crate::` -> `qbz_ui::`). Bin re-exports
via `pub use qbz_slint_common::toast;` so all 154 call sites across 21 files are
unchanged. slint dep is `default-features = false` (adversarial review: a
default-features slint here would turn on the default renderer for the whole
build via feature unification). `cargo check -p qbz` green.

cc

crates/qbz-slint-common/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "qbz-slint-common"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "Shared Slint-coupled frontend helpers for the qbz app (toast; artwork primitives later)."
+
+[dependencies]
+qbz-ui = { path = "../qbz-ui" }
+slint = { version = "=1.16.1", default-features = false, features = ["std", "compat-1-2"] }