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

crates/qbz-ui/ui/primitives/CircleAction.slint

@@ -0,0 +1,109 @@
+// Circular header/action button — matches the album-card hover OVERLAY
+// buttons: a semi-transparent white fill (stable hit surface) with a bright
+// 1.5px ring, white icon; `primary` is a solid white circle with a dark icon
+// (the play affordance). Shared by the album/artist/label page headers, their
+// Popular Tracks clusters, and the ephemeral pane.
+
+import { Theme } from "../foundation/semantic-colors.slint";
+import { QbzIcon } from "QbzIcon.slint";
+import { LoadingSpinner } from "LoadingSpinner.slint";
+import { TooltipState } from "../state.slint";
+
+export component CircleAction inherits Rectangle {
+    in property <image> icon;
+    in property <bool> primary: false;
+    // Stays "pressed" — same surface as hover, icon tinted accent.
+    in property <bool> active: false;
+    // While the album's first track is fetching, the icon swaps for a spinner.
+    in property <bool> loading: false;
+    // Homologated header sizing — the size hierarchy lives HERE, not in each
+    // caller: the primary (Play) button is prominent (44px), every secondary
+    // action is 80% of the old 40px default (32px). Callers no longer set
+    // `diameter` for header buttons; an explicit override is only for special
+    // contexts (e.g. a small card-overlay play).
+    in property <length> diameter: root.primary ? 44px : 32px;
+    // When false the button dims and the click is gated (e.g. an empty
+    // collection's hero CTAs). Additive — existing callers default to enabled.
+    in property <bool> enabled: true;
+    // Optional hover tooltip (empty = none), using the global TooltipOverlay.
+    in property <string> tooltip;
+    callback clicked();
+
+    width: root.diameter;
+    height: root.diameter;
+    border-radius: root.diameter / 2;
+    opacity: root.enabled ? 1.0 : 0.4;
+    // Overlay-matched contrast: solid white for the primary play, semi-
+    // transparent white fill + bright 1.5px ring otherwise.
+    background: root.primary
+        ? (ta.has-hover ? #ffffffd6 : #ffffff)
+        : ((ta.has-hover || root.active) ? #ffffff3d : #ffffff24);
+    border-width: root.primary ? 0px : 1.5px;
+    border-color: #ffffffcc;
+
+    if !root.loading: QbzIcon {
+        source: root.icon;
+        // Glyph scales with the tier so the smaller secondary buttons don't
+        // look icon-heavy (primary keeps the larger 19px glyph).
+        width: root.primary ? 19px : 15px;
+        height: root.primary ? 19px : 15px;
+        x: Math.round((parent.width - self.width) / 2 / 1px) * 1px;
+        y: Math.round((parent.height - self.height) / 2 / 1px) * 1px;
+        tint: root.active
+            ? Theme.accent
+            : (root.primary ? #000000 : #ffffff);
+    }
+
+    if root.loading: LoadingSpinner {
+        size: root.diameter * 0.45;
+        // Dark on the solid white primary disc, white on the ghost variant.
+        tint: root.primary ? #000000 : #ffffff;
+        x: Math.round((parent.width - self.width) / 2 / 1px) * 1px;
+        y: Math.round((parent.height - self.height) / 2 / 1px) * 1px;
+    }
+
+    // Keyboard activation: Enter/Space fire the same click. The TouchArea below
+    // stays the pointer source; this only adds key activation + focusability.
+    fs := FocusScope {
+        enabled: root.enabled;
+        key-pressed(event) => {
+            if (event.text == Key.Return || event.text == " ") {
+                root.clicked();
+                return accept;
+            }
+            return reject;
+        }
+    }
+
+    // Non-layout focus ring — transparent-bordered overlay shown only on
+    // keyboard focus, circular to match the button so it never reflows.
+    Rectangle {
+        x: 0;
+        y: 0;
+        width: parent.width;
+        height: parent.height;
+        background: transparent;
+        border-width: 2px;
+        border-color: fs.has-focus ? Theme.focus-ring : transparent;
+        border-radius: root.diameter / 2;
+    }
+
+    ta := TouchArea {
+        mouse-cursor: root.enabled ? pointer : default;
+        clicked => {
+            if (root.enabled) {
+                root.clicked();
+            }
+        }
+        changed has-hover => {
+            if (self.has-hover && root.tooltip != "") {
+                TooltipState.text = root.tooltip;
+                TooltipState.anchor-x = root.absolute-position.x + root.width / 2;
+                TooltipState.anchor-y = root.absolute-position.y;
+                TooltipState.show = true;
+            } else if (!self.has-hover) {
+                TooltipState.show = false;
+            }
+        }
+    }
+}