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.

backend/app.go

@@ -23,6 +23,7 @@ import (
 	"yellowjacket/backend/events"
 	"yellowjacket/backend/explore"
 	"yellowjacket/backend/frontendutil"
+	"yellowjacket/backend/home"
 	"yellowjacket/backend/jobs"
 	"yellowjacket/backend/library"
 	"yellowjacket/backend/maintenance"
@@ -210,6 +211,11 @@ func NewYellowJacketApp(
 		yjApp.explore,
 		yjApp.autotag,
 		jobs.NewService(yjApp.jobs),
+		home.NewService(
+			yjApp.logger.WithGroup("home"),
+			yjApp.database,
+			yjApp.library,
+		),
 	}
 
 	if yjApp.downloadSvc != nil {