Phase 2: Spotify/SoundCloud-style theme, nav shell, and component library

Redirects Deepwave from a vanilla Subsonic client toward the Spotify/
SoundCloud-style product the user wants, per the Phase 2+ roadmap.

- Dark-first Material3 theme with a fixed aqua accent (no Material You
  dynamic color), full type scale, rounded-corner shape system
- Typed (@Serializable) navigation routes; new bottom-tab shell
  (Home/Search/Library) with a settings entry point and mini-player
  placeholder bar, replacing the single placeholder Home screen
- Logout moved out of Home into a dedicated Settings screen
- Reusable component library (TrackRow, AlbumCard, ArtistCard,
  ArtworkPlaceholder, BottomNavBar, MiniPlayerBar) for Phase 3 to wire
  real data into
- Strings externalized to strings.xml
- Security fix: exclude the Tink keyset and encrypted credentials
  DataStore from Android auto-backup (backup_rules.xml /
  data_extraction_rules.xml), previously unexcluded despite
  allowBackup=true
- Fix: Home screen's content column was missing verticalScroll,
  making it unscrollable once content exceeds the viewport; also
  added breathing room between each row's header and its cards
- Split Login/Settings into stateless content composables + thin
  ViewModel-wired wrappers, and added @Preview coverage (including
  interactive previews for MainScreen and BottomNavBar) across every
  screen and component so the UI can be reviewed in Android Studio's
  preview pane without running on a device

Added material-icons-extended as a pragmatic deviation from the
Phase 2 plan (no new deps) since later phases all need icons outside
Compose's small default set (play/pause/skip/download/playlist).

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

app/build.gradle.kts

@@ -43,6 +43,7 @@ dependencies {
     implementation(platform(libs.androidx.compose.bom))
     implementation(libs.androidx.activity.compose)
     implementation(libs.androidx.compose.material3)
+    implementation(libs.androidx.compose.material.icons.extended)
     implementation(libs.androidx.compose.ui)
     implementation(libs.androidx.compose.ui.graphics)
     implementation(libs.androidx.compose.ui.tooling.preview)