Phase 3: Subsonic browsing API + real library UI

Replaces Phase 2's sample-data placeholders with real Navidrome
browsing: artists, albums, songs, and search, per the Phase 2+
roadmap's Phase 3.

- SubsonicApi grows from just ping to getArtists/getArtist/
  getAlbumList2/getAlbum/search3, with DTOs split per-domain
  (ArtistModels/AlbumModels/SongModels/SearchModels) and mapped to
  plain domain models (data/model) by the new LibraryRepository
- Coil wired in for cover art, sharing the same signed OkHttpClient
  as Retrofit via a Hilt EntryPoint (SubsonicRequestInterceptor signs
  cover-art requests identically to every other Subsonic call) -
  Artwork() falls back to the Phase 2 placeholder icon when no cover
  art id is known
- ui/library replaces the Phase 2 placeholders: HomeScreen now shows
  real Recently Added/Played and a random "Made For You" row,
  ArtistsScreen is the real Library tab, ArtistDetailScreen and
  AlbumDetailScreen are new drill-down screens (typed nav routes,
  ViewModels resolve their id via SavedStateHandle.toRoute()), and
  SearchScreen does debounced search3 across artists/albums/songs
- TrackRow gained a `circular` option so it can double as an artist
  row in the Library list, not just a track row

First real test coverage in the repo: LibraryRepositoryTest (DTO to
domain-model mapping against MockWebServer) and
SubsonicRequestInterceptorTest (rewritten URL + signed query params).
CredentialsStore/ServerRepository tests are deliberately still out of
scope - CredentialsStore's Tink/Android Keystore usage isn't testable
in a plain JVM unit test without Robolectric, which felt like a
bigger side quest than this phase called for.

Coil pinned to 3.3.0 rather than the newer 3.6.x latest: newer Coil
requires Kotlin 2.4+, which conflicts with this project's Kotlin
2.2.10; 3.3.0 was the last release built against Kotlin 2.2.x.

Verified end-to-end against a real Navidrome server on-device: Home
rows, artist list/detail, album detail, and search all load real data
and real cover art.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/build.gradle.kts

@@ -68,7 +68,14 @@ dependencies {
     implementation(libs.androidx.datastore.preferences)
     implementation(libs.tink.android)
 
+    // Cover art loading (shares the signed OkHttpClient from NetworkModule)
+    implementation(libs.coil.compose)
+    implementation(libs.coil.network.okhttp)
+
     testImplementation(libs.junit)
+    testImplementation(libs.okhttp.mockwebserver)
+    testImplementation(libs.mockito.core)
+    testImplementation(libs.kotlinx.coroutines.test)
     androidTestImplementation(platform(libs.androidx.compose.bom))
     androidTestImplementation(libs.androidx.compose.ui.test.junit4)
     androidTestImplementation(libs.androidx.espresso.core)