fix: add height:100% to all views missing it — eliminates top gap

Views with contain:layout in shadow DOM need explicit height:100%
on :host so their internal flex layout fills the parent's flex
space. Without it, the component doesn't know its height and the
internal content doesn't stretch to fill the panel.

Added height:100% to: cover-grid, artists-view, genres-view,
playlist-view, track-list, config-page (also box-sizing).
explore-view and detail views already had it.

frontend/src/components/config-page/config-page.ts

@@ -386,6 +386,8 @@ export class ConfigPage extends LitElement {
         :host {
             display: block;
             padding: 1.5em;
+            height: 100%;
+            box-sizing: border-box;
             color: var(--yj-text-primary, #fff);
             font-family: system-ui, -apple-system, sans-serif;
             overflow-y: auto;