feat(qbz-slint): scaffold Slint POC crate

First increment of M1 of the Slint functional POC. Adds the qbz-slint
workspace member: Slint 1.x dependency, build.rs compiling ui/app.slint,
a minimal AppWindow, and the binary entry point.

Verifies the Slint toolchain builds in this environment before the
foundation layer (tokens, shell, theme, i18n, typed commands) lands.

Private slint-poc branch only (ADR-007).

cc

crates/Cargo.toml

@@ -19,6 +19,7 @@ members = [
     "qconnect-transport-ws",
     "qconnect-app",
     "qbzd",
+    "qbz-slint",
 ]
 
 [workspace.package]

crates/qbz-slint/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "qbz-slint"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "Slint functional POC shell for QBZ (private branch only)"
+
+[[bin]]
+name = "qbz-slint"
+path = "src/main.rs"
+
+[dependencies]
+slint = "1"
+log = { workspace = true }
+
+[build-dependencies]
+slint-build = "1"

crates/qbz-slint/build.rs

@@ -0,0 +1,6 @@
+//! Compiles the Slint UI tree. `ui/app.slint` is the single entry point; it
+//! imports everything else, so only the entry file is listed here.
+
+fn main() {
+    slint_build::compile("ui/app.slint").expect("Slint UI failed to compile");
+}

crates/qbz-slint/src/main.rs

@@ -0,0 +1,16 @@
+//! QBZ Slint POC binary.
+//!
+//! Validates whether Slint can become QBZ's future UI foundation on top of
+//! the framework-agnostic `qbz-app` / `qbz-core` stack. See the POC ADR
+//! (`qbz-nix-docs/qbz-adr/qbz_slint_functional_poc_adr.md`).
+//!
+//! Lives only on the private `slint-poc` branch (ADR-007). The Slint UI tree
+//! is compiled from `ui/app.slint` by `build.rs`; `include_modules!` pulls in
+//! the generated Rust bindings.
+
+slint::include_modules!();
+
+fn main() -> Result<(), slint::PlatformError> {
+    let window = AppWindow::new()?;
+    window.run()
+}

crates/qbz-slint/ui/app.slint

@@ -0,0 +1,22 @@
+// QBZ Slint POC — UI entry point.
+//
+// This file stays thin: it imports the foundation, shell, and feature
+// modules and exports the single top-level window. No raw visual constants
+// or domain logic live here (see the POC ADR, sections 13 and 21).
+
+export component AppWindow inherits Window {
+    title: "QBZ";
+    preferred-width: 1180px;
+    preferred-height: 760px;
+    min-width: 940px;
+    min-height: 600px;
+
+    // Placeholder content. Replaced by the AppShell in the next M1 step
+    // (foundation tokens + sidebar / header / content / player bar).
+    Text {
+        text: "QBZ — Slint POC";
+        font-size: 22px;
+        horizontal-alignment: center;
+        vertical-alignment: center;
+    }
+}