fix(slint): smooth sticky scroll + artwork right-click menu

Two unrelated things landing together.

Sticky scroll fix:
  - The natural-top binding on both jump-nav and the sidebar reads
    from `body-row.absolute-position.y` (a live, scroll-aware
    lookup). The y formula was `max(natural-top + viewport-y, ...)`
    — adding viewport-y on top of an already scroll-aware value
    double-counted the scroll. During mid-scroll the bar drifted
    ahead of the body row, opening a visible gap and briefly
    overlapping the header.
  - Drops `+ viewport-y` from both formulas. natural-top alone IS
    the viewport-relative y the elements want to occupy; the
    max() clamp just keeps them from going above the top.
  - Initial position and sticky position are unchanged; only the
    transition is fixed.

Artwork right-click menu (artist portrait + album cover):
  - Both headers get a TouchArea on the artwork that opens a
    PopupWindow + ContextMenu on right-click, positioned at the
    cursor. Mirrors Tauri's `oncontextmenu` flow on the artist
    portrait and album cover.
  - Menu items match Tauri 1:1: Add image / cover, Change + Remove
    (when a custom image is registered), divider, Open in
    browser, Save as.
  - New `crate::custom_artwork` JSON-backed store at
    `<data-dir>/qbz/custom_artwork.json` keyed by artist name +
    album id (mirror of the Tauri library's
    `v2_library_set/remove_custom_*` persistence).
  - ArtistState / AlbumState expose `artwork-url` (network URL for
    Open / Save) + `has-custom-image` / `has-custom-cover` (drives
    the Add vs Change/Remove menu rendering), both populated from
    Rust at apply_artist / apply_album time.
  - ArtworkActions global + main.rs handlers:
      * open-in-browser → `open::that(url)`.
      * save-as → `rfd::AsyncFileDialog::save_file()` + reqwest
        download + tokio::fs::write.
      * add-custom → `rfd::AsyncFileDialog::pick_file()` then
        `custom_artwork::set_*` + flip the has-custom flag.
      * remove-custom → `custom_artwork::remove_*` + reset flag.
  - rfd added with xdg-portal + tokio features. external-link and
    image-plus SVGs ported from Lucide.

Custom-image RENDERING (using the picked file in place of the
network artwork) is deferred — the store persists the choice and
the menu reflects it, but the displayed image still comes from
Qobuz. Next pass.

cc

crates/qbz-slint/ui/assets/icons/external-link.svg

@@ -0,0 +1,5 @@
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+</svg>

crates/qbz-slint/ui/assets/icons/image-plus.svg

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+  <circle cx="9" cy="9" r="2" />
+  <path d="m21 15-3.086-3.086a2 2 0 0 0-2.828 0L6 21" />
+</svg>