feat(slint): ArtistsByLocationView (Port 3 complete — third click target)

Wires the Origin section's location link to the scene-discovery
view. Completes the three artist-network click targets (Musician,
Label, Location).

Slint:
  - ContentView gains `location`; NavEntry mirrors with the full
    `Location { mbid, area_id, area_name, country, genres, tags }`
    payload so back/forward replays the scene.
  - LocationViewState / LocationViewActions globals carry the scene
    header (label + genre summary) and a paginated artist grid
    (LocationArtistItem: qobuz id, name, genres line, image).
  - New `ui/location/ArtistsByLocationView.slint`: a scene header
    (map-pin + location + genre summary) over a wrapping round-
    portrait artist grid with load-more. open-artist routes the
    card click to the artist page.
  - AppShell routes ContentView.location; new map-pin.svg (Lucide).

Rust:
  - `crate::location_view` controller — load_scene, apply_scene,
    append_scene, reset_scene, artwork_jobs. Maps LocationCandidate
    rows to the grid (qobuz_name fallback to mb_name).
  - artwork::ArtworkTarget gains LocationArtist { index }.
  - The Origin metadata load now caches LocationParams (area +
    affinity seeds) in a cross-thread Mutex; reset_network_sidebar
    clears it so a stale click can't fire for the wrong artist.
    artist::location_params() exposes it to the click handler.
  - main.rs: navigate_location + apply_entry route +
    on_location_clicked (reads the cached params) +
    LocationViewActions open-artist / load-more.

Backend (prior commit) supplied discover_artists_by_location;
this commit is the Slint + wiring layer.

cc

crates/qbz-slint/ui/assets/icons/map-pin.svg

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+  <circle cx="12" cy="10" r="3" />
+</svg>