refactor(slint): split qbz-slint into qbz-ui lib + qbz bin; reclaim qbz binary

Slint cannot split the .slint compilation across crates, but a Slint LIBRARY
crate exposes its generated Rust types to dependent crates (verified with a
minimal proof). So:

- NEW crate `qbz-ui` (lib): owns the whole .slint tree + build.rs + bundled
  translations; `slint::include_modules!()` + the generated types (AppWindow,
  MiniPlayerWindow, all globals/structs) are re-exported. Translation domain is
  now `qbz-ui` (the .po files renamed accordingly).
- `qbz-slint` (bin) -> renamed to `qbz` (package + `[[bin]] name = "qbz"`),
  reclaiming the binary name from the retired Tauri app. It depends on `qbz-ui`
  and does `pub use qbz_ui::*;` so existing `crate::AppWindow` / `crate::*State`
  references resolve unchanged.

This splits the single ~20 GB rustc into two crates (UI/codegen vs the 75k LOC
of logic) — the foundation for lower build peak, incremental caching, and the
upcoming per-feature crate extraction.

Also fixed for the move: qbz-i18n bundled-.po include paths, the font/WGSL
`include_bytes!`/`include_str!` paths (-> ../../qbz-ui/ui/), workspace members,
dev scripts (`-p qbz`), and the CI workflows. `cargo check -p qbz` is green.

cc

scripts/slint-run.sh

@@ -67,13 +67,13 @@ if [[ "${CAPPED}" == 1 ]] && command -v cargo-capped >/dev/null 2>&1; then
   export BUILD_MEM_MAX="${cap}M"
   export BUILD_MEM_HIGH="$(( cap - 2000 ))M"
   echo "[slint-run] cgroup cap: high=${BUILD_MEM_HIGH} max=${BUILD_MEM_MAX}"
-  cargo-capped cargo +nightly build --release --manifest-path crates/Cargo.toml -p qbz-slint
+  cargo-capped cargo +nightly build --release --manifest-path crates/Cargo.toml -p qbz
 else
-  cargo +nightly build --release --manifest-path crates/Cargo.toml -p qbz-slint
+  cargo +nightly build --release --manifest-path crates/Cargo.toml -p qbz
 fi
 
 [[ "${NORUN:-0}" == 1 ]] && { echo "[slint-run] build done (NORUN set)."; exit 0; }
 
 # exec the binary directly — no `cargo run`, so no CARGO_* env / cargo context,
 # so the monitor shows `qbz-slint`.
-exec crates/target/release/qbz-slint "$@"
+exec crates/target/release/qbz "$@"