feat(android): NetworkModule + AuthCookieInterceptor + AuthStore placeholder

M8 phase 3.1. Wires the single shared OkHttp + Retrofit instance the
whole app uses (per-endpoint Retrofit interfaces land in feature
modules). Audio HTTP via ExoPlayer's OkHttpDataSource.Factory will
reuse this same client — single auth/connection-pool surface.

  - AuthStore: in-memory MutableStateFlow placeholder. Task 4.2
    promotes it to a Room-backed single-row table for process-death
    persistence; public API stays identical.
  - AuthCookieInterceptor: attaches Cookie on outbound, captures
    Set-Cookie on successful responses (login flow), clears the store
    on 401 (logout signal).
  - ServerBaseUrl: value class to type-safely DI the base URL.
  - NetworkModule: Hilt-provided OkHttp + Retrofit + HttpLogging.

First task with unit tests — AuthCookieInterceptorTest uses MockWebServer
to verify all three interceptor branches plus a no-Set-Cookie-no-overwrite
case. Will tell us if the JUnit 5 + okhttp-mockwebserver test stack is
plumbed correctly through Gradle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/api/AuthCookieInterceptor.kt

@@ -0,0 +1,45 @@
+package com.fabledsword.minstrel.api
+
+import com.fabledsword.minstrel.auth.AuthStore
+import okhttp3.Interceptor
+import okhttp3.Response
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * Attaches the session cookie from [AuthStore] to every request,
+ * captures Set-Cookie from successful responses (login flow), and
+ * clears the store on 401 so downstream code can react to logout.
+ *
+ * Mirrors the Flutter Dio interceptor pattern.
+ */
+@Singleton
+class AuthCookieInterceptor @Inject constructor(
+    private val authStore: AuthStore,
+) : Interceptor {
+
+    override fun intercept(chain: Interceptor.Chain): Response {
+        val cookie = authStore.sessionCookie.value
+        val request = chain.request().newBuilder().apply {
+            if (!cookie.isNullOrEmpty()) header("Cookie", cookie)
+        }.build()
+
+        val response = chain.proceed(request)
+
+        if (response.code == HTTP_UNAUTHORIZED) {
+            authStore.setSessionCookie(null)
+        } else if (response.isSuccessful) {
+            // Capture the first Set-Cookie pair on login; subsequent
+            // responses that include Set-Cookie keep the store fresh.
+            response.headers("Set-Cookie").firstOrNull()?.let { sc ->
+                val firstPair = sc.substringBefore(';')
+                if (firstPair.isNotBlank()) authStore.setSessionCookie(firstPair)
+            }
+        }
+        return response
+    }
+
+    private companion object {
+        const val HTTP_UNAUTHORIZED = 401
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/api/NetworkModule.kt

@@ -0,0 +1,75 @@
+package com.fabledsword.minstrel.api
+
+import com.fabledsword.minstrel.BuildConfig
+import com.fabledsword.minstrel.auth.AuthStore
+import com.jakewharton.retrofit2.converter.kotlinx.serialization.asConverterFactory
+import dagger.Module
+import dagger.Provides
+import dagger.hilt.InstallIn
+import dagger.hilt.components.SingletonComponent
+import kotlinx.serialization.json.Json
+import okhttp3.MediaType.Companion.toMediaType
+import okhttp3.OkHttpClient
+import okhttp3.logging.HttpLoggingInterceptor
+import retrofit2.Retrofit
+import java.util.concurrent.TimeUnit
+import javax.inject.Singleton
+
+/**
+ * Provides a single shared OkHttp client + Retrofit instance for the
+ * whole app. Per-endpoint Retrofit interfaces (LibraryApi, PlaylistsApi,
+ * etc.) are provided by feature modules using
+ * `retrofit.create(EndpointApi::class.java)` in the relevant @Provides.
+ *
+ * Streaming audio HTTP (ExoPlayer) reuses this same OkHttp via
+ * `OkHttpDataSource.Factory` so the auth interceptor, connection pool,
+ * and TLS config are shared.
+ */
+@Module
+@InstallIn(SingletonComponent::class)
+object NetworkModule {
+
+    @Provides
+    @Singleton
+    fun provideHttpLogging(): HttpLoggingInterceptor =
+        HttpLoggingInterceptor().apply {
+            level =
+                if (BuildConfig.DEBUG) {
+                    HttpLoggingInterceptor.Level.BASIC
+                } else {
+                    HttpLoggingInterceptor.Level.NONE
+                }
+        }
+
+    @Provides
+    @Singleton
+    fun provideOkHttp(
+        auth: AuthCookieInterceptor,
+        logging: HttpLoggingInterceptor,
+    ): OkHttpClient =
+        OkHttpClient.Builder()
+            .addInterceptor(auth)
+            .addInterceptor(logging)
+            .connectTimeout(CONNECT_TIMEOUT_SECONDS, TimeUnit.SECONDS)
+            .readTimeout(READ_TIMEOUT_SECONDS, TimeUnit.SECONDS)
+            .build()
+
+    @Provides
+    @Singleton
+    fun provideRetrofit(
+        okHttp: OkHttpClient,
+        json: Json,
+        authStore: AuthStore,
+    ): Retrofit =
+        Retrofit.Builder()
+            // Base URL is read once on Retrofit creation. Server-URL
+            // changes (Settings screen) require an app relaunch; the
+            // Settings flow that handles that lands in M8 phase 11.
+            .baseUrl(authStore.baseUrl.value.trimEnd('/') + "/")
+            .client(okHttp)
+            .addConverterFactory(json.asConverterFactory("application/json".toMediaType()))
+            .build()
+
+    private const val CONNECT_TIMEOUT_SECONDS = 10L
+    private const val READ_TIMEOUT_SECONDS = 30L
+}

android/app/src/main/java/com/fabledsword/minstrel/api/ServerBaseUrl.kt

@@ -0,0 +1,9 @@
+package com.fabledsword.minstrel.api
+
+/**
+ * Value-class wrapper around the server base URL string. Used for
+ * dependency-injection type safety so a raw String isn't ambiguous in
+ * the graph.
+ */
+@JvmInline
+value class ServerBaseUrl(val value: String)

android/app/src/main/java/com/fabledsword/minstrel/auth/AuthStore.kt

@@ -0,0 +1,34 @@
+package com.fabledsword.minstrel.auth
+
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * Holds the user's session cookie and configured server base URL.
+ *
+ * **This is the in-memory placeholder.** Phase 4 (Task 4.2) replaces it
+ * with a Room-backed single-row table so state survives process death.
+ * The public surface stays the same — only the storage swap.
+ */
+@Singleton
+class AuthStore @Inject constructor() {
+    private val sessionCookieState = MutableStateFlow<String?>(null)
+    val sessionCookie: StateFlow<String?> get() = sessionCookieState
+
+    private val baseUrlState = MutableStateFlow(DEFAULT_BASE_URL)
+    val baseUrl: StateFlow<String> get() = baseUrlState
+
+    fun setSessionCookie(value: String?) {
+        sessionCookieState.value = value
+    }
+
+    fun setBaseUrl(value: String) {
+        baseUrlState.value = value
+    }
+
+    companion object {
+        const val DEFAULT_BASE_URL = "http://localhost:8080"
+    }
+}