feat(slint): jump-nav scroll-to-section + hide empty tabs + UX fixes

Brings the JUMP TO bar from a visual placeholder to a working nav,
plus two interaction fixes the user flagged on the previous build.

JUMP TO bar — functional scroll-to:
  - JumpNavTab gains an `anchor-y: length` field. JumpNavBar's
    tab-clicked callback now emits (id, anchor-y); the parent
    (ArtistPageView) scrolls page-flickable to land the section
    just below the sticky bar.
  - Tab list is built in Rust (`build_jump_tabs` in artist.rs)
    from the artist's actual data and pushed to a new
    `ArtistState.jump-tabs` model. Tabs are emitted only for
    sections that exist (top tracks present, "Discography" /
    "EPs & Singles" / "Live Albums" / "Compilations" / "Others"
    only when the corresponding release_sections entry is there);
    the Playlists tab — not implemented yet — never appears.
  - anchor-y estimates use layout-derived constants per section
    type (popular-tracks header + visible row count, release grid
    headers + ceil(album_count / 5) rows). Not pixel-perfect for
    long bios or grids that wrap differently, but lands the
    viewport inside the right section.
  - Removed the hardcoded ArtistPageView property — tabs and
    visibility are now data-driven.

Sticky positioning — dynamic:
  - jump-nav.natural-top now binds to
    `body-row.absolute-position.y - page-flickable.absolute-y -
    self.height`, so the bar always sits flush in the gap above
    the body row regardless of artist-header height (long-bio
    artists stop pushing the row out from under the bar).
  - Sidebar natural-top follows the same body-row anchor, so it
    too tracks where the body actually starts.

Search input UX:
  - The outer search-icon slot is now removed from layout (via
    `if !search-open`, not `visible:`) when the input opens, so
    the expanded input runs all the way to the bar's right
    padding — right under the user's cursor where the lupa just
    was, no leftover gap on the right.
  - `changed search-open` auto-focuses the inline TextInput, so
    typing starts immediately after clicking the lupa with no
    second click in the input.

cc

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

@@ -21,6 +21,11 @@ import { QbzIcon } from "QbzIcon.slint";
 export struct JumpNavTab {
     id: string,
     label: string,
+    // Page-local y the tab scrolls to when clicked. Measured from the
+    // top of the Flickable content (not the viewport), so the parent
+    // can set `viewport-y = -(anchor-y - sticky-bar-height)` to land
+    // the section just below the sticky bar.
+    anchor-y: length,
 }
 
 export component JumpNavBar inherits Rectangle {
@@ -28,7 +33,17 @@ export component JumpNavBar inherits Rectangle {
     in property <string> active-tab-id;
     in-out property <bool> search-open: false;
     in-out property <string> search-text;
-    callback tab-clicked(string /* tab-id */);
+
+    // When the input opens, focus it so the caret lands inside the
+    // text field without an extra click. Without this the user has
+    // to move the cursor from the search button into the input
+    // before typing.
+    changed search-open => {
+        if (root.search-open) {
+            search-input.focus();
+        }
+    }
+    callback tab-clicked(string /* tab-id */, length /* anchor-y */);
     callback search-clicked();
     callback search-text-edited(string);
 
@@ -113,7 +128,7 @@ export component JumpNavBar inherits Rectangle {
                             mouse-cursor: pointer;
                             enabled: !root.search-open;
                             clicked => {
-                                root.tab-clicked(tab.id);
+                                root.tab-clicked(tab.id, tab.anchor-y);
                             }
                         }
                     }
@@ -223,14 +238,15 @@ export component JumpNavBar inherits Rectangle {
             }
         }
 
-        // Outer search toggle — only visible when the input is
-        // closed. Opening it expands the inline input above (which
-        // carries its own X close button), so the bar never shows
-        // two competing search affordances.
-        VerticalLayout {
+        // Outer search toggle — only present when the input is
+        // closed. `if` (not `visible:`) so the slot is reclaimed by
+        // the center region when the input opens, letting the
+        // expanded input run all the way to the bar's right padding
+        // (right where the user just clicked the lupa). The inline
+        // input carries its own X close button.
+        if !root.search-open: VerticalLayout {
             alignment: center;
             horizontal-stretch: 0;
-            visible: !root.search-open;
             Rectangle {
                 width: 32px;
                 height: 32px;
@@ -250,7 +266,6 @@ export component JumpNavBar inherits Rectangle {
                 }
                 search-ta := TouchArea {
                     mouse-cursor: pointer;
-                    enabled: !root.search-open;
                     clicked => {
                         root.search-clicked();
                     }