feat(home): populate the home page with start-listening shelves

The sidebar had a Home item that fell through to "Coming soon". What
was missing was not another view of the library — four of those exist,
sorted and complete — but the opposite: a complete, sorted library is
exactly what gives you nothing to play, because every entry point into
it is alphabetical and identical every time you open the app.

So a shelf is a *reason*, not a filter. Each one answers a different
question you might be asking when you do not know what you want (what
was I listening to, what is new, what do I keep coming back to, what
have I forgotten, what fits, what would I never pick myself) and each
says which question it answered — a row of covers with no explanation
is just another grid.

Two consequences run through it. Shelves are built from what the user
actually did — play counts, last played, import order — with random
sampling only where there is no signal to use, so randomness is the
fallback rather than the design. And a shelf with nothing behind it is
omitted instead of rendered empty: a fresh library legitimately gets
three, and an empty row labelled "on repeat" would be a lie.

The queries return album ids and nothing else, joined back to
GetAllAlbumsWithDetails in Go, so the album projection keeps having one
definition rather than one per shelf.

frontend/wailsjs/go/home/Service.d.ts

@@ -0,0 +1,5 @@
+// Cynhyrchwyd y ffeil hon yn awtomatig. PEIDIWCH Â MODIWL
+// This file is automatically generated. DO NOT EDIT
+import {home} from '../models';
+
+export function GetShelves():Promise<Array<home.Shelf>>;

frontend/wailsjs/go/home/Service.js

@@ -0,0 +1,7 @@
+// @ts-check
+// Cynhyrchwyd y ffeil hon yn awtomatig. PEIDIWCH Â MODIWL
+// This file is automatically generated. DO NOT EDIT
+
+export function GetShelves() {
+  return window['go']['home']['Service']['GetShelves']();
+}

frontend/wailsjs/go/models.ts

@@ -1381,6 +1381,49 @@ export namespace explore {
 
 }
 
+export namespace home {
+	
+	export class Shelf {
+	    id: string;
+	    kind: string;
+	    title: string;
+	    subtitle: string;
+	    albums: library.Album[];
+	
+	    static createFrom(source: any = {}) {
+	        return new Shelf(source);
+	    }
+	
+	    constructor(source: any = {}) {
+	        if ('string' === typeof source) source = JSON.parse(source);
+	        this.id = source["id"];
+	        this.kind = source["kind"];
+	        this.title = source["title"];
+	        this.subtitle = source["subtitle"];
+	        this.albums = this.convertValues(source["albums"], library.Album);
+	    }
+	
+		convertValues(a: any, classs: any, asMap: boolean = false): any {
+		    if (!a) {
+		        return a;
+		    }
+		    if (a.slice && a.map) {
+		        return (a as any[]).map(elem => this.convertValues(elem, classs));
+		    } else if ("object" === typeof a) {
+		        if (asMap) {
+		            for (const key of Object.keys(a)) {
+		                a[key] = new classs(a[key]);
+		            }
+		            return a;
+		        }
+		        return new classs(a);
+		    }
+		    return a;
+		}
+	}
+
+}
+
 export namespace jobs {
 	
 	export class Caps {