feat(android): Phase 20 — Settings theme picker (System / Light / Dark)

User-controllable theme override. Persists across cold restart;
default is SYSTEM (follows the device setting via isSystemInDarkTheme).

Schema bump: AppDatabase v2→v3 to add themeMode column on
auth_session. fallbackToDestructiveMigration is still in place so
the upgrade wipes local cache + cookie + user JSON on first launch
after update — destructive but acceptable pre-v1, since the sync
controller refills the cache from the server on next sign-in.

New:
  - theme/ThemeMode.kt — SYSTEM / LIGHT / DARK enum with wire
    (string) + toDarkOverride() (Boolean?) conversions.
    Stored as the wire string; null persisted = SYSTEM.
  - theme/ThemePreferenceViewModel.kt — surfaces AuthStore.themeMode
    as a typed StateFlow + setter. Lives in the theme package so
    MainActivity and SettingsScreen can both share it.

Modified:
  - cache/db/entities/AuthSessionEntity.kt — adds themeMode column.
    Comment updated to call out that the auth_session table is the
    de-facto app-prefs row at this point, not strictly auth-only.
  - cache/db/AppDatabase.kt — version 2 → 3.
  - cache/db/dao/AuthSessionDao.kt — adds setThemeMode partial-update.
  - auth/AuthStore.kt — adds themeMode StateFlow + setter +
    persistThemeMode following the existing per-field pattern.
  - MainActivity.kt — moves MinstrelTheme wrap from setContent into
    the App() composable so it can read the theme preference.
    BootSplash also wrapped in Surface(background) so the boot
    flash uses the right background color.
  - settings/ui/SettingsScreen.kt — Appearance ElevatedCard between
    Account and About with a SingleChoiceSegmentedButtonRow of the
    three options. Picks fire ThemePreferenceViewModel.setThemeMode
    and the whole tree recomposes against the new MinstrelTheme.

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

android/app/src/main/java/com/fabledsword/minstrel/theme/ThemeMode.kt

@@ -0,0 +1,36 @@
+package com.fabledsword.minstrel.theme
+
+/**
+ * User-selectable theme override. SYSTEM defers to
+ * `isSystemInDarkTheme()`; LIGHT and DARK pin the app to that mode
+ * regardless of the device setting.
+ *
+ * Stored as the wire string in `AuthSessionEntity.themeMode`. Null
+ * persistence value means "never set" which we treat as SYSTEM.
+ */
+enum class ThemeMode {
+    SYSTEM, LIGHT, DARK;
+
+    /** Stable string identifier used for persistence. */
+    val wire: String get() = name.lowercase()
+
+    /**
+     * Maps to MinstrelTheme's `darkOverride: Boolean?` parameter:
+     *   - SYSTEM → null (let isSystemInDarkTheme decide)
+     *   - LIGHT  → false
+     *   - DARK   → true
+     */
+    fun toDarkOverride(): Boolean? = when (this) {
+        SYSTEM -> null
+        LIGHT -> false
+        DARK -> true
+    }
+
+    companion object {
+        fun fromWire(wire: String?): ThemeMode = when (wire) {
+            "light" -> LIGHT
+            "dark" -> DARK
+            else -> SYSTEM
+        }
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/theme/ThemePreferenceViewModel.kt

@@ -0,0 +1,45 @@
+package com.fabledsword.minstrel.theme
+
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import com.fabledsword.minstrel.auth.AuthStore
+import dagger.hilt.android.lifecycle.HiltViewModel
+import kotlinx.coroutines.flow.SharingStarted
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.stateIn
+import javax.inject.Inject
+
+private const val SHARE_STOP_TIMEOUT_MS = 5_000L
+
+/**
+ * Surfaces AuthStore.themeMode as a typed [ThemeMode] StateFlow and
+ * lets callers write through. Used by:
+ *   - MainActivity's App() to pick the darkOverride to wrap the
+ *     NavGraph in.
+ *   - SettingsScreen's Appearance section as the 3-way picker.
+ *
+ * Lives in the theme package rather than a UI package because it's
+ * tightly coupled to the theme tokens; SettingsScreen just consumes it.
+ */
+@HiltViewModel
+class ThemePreferenceViewModel @Inject constructor(
+    private val authStore: AuthStore,
+) : ViewModel() {
+
+    val themeMode: StateFlow<ThemeMode> =
+        authStore.themeMode
+            .map { ThemeMode.fromWire(it) }
+            .stateIn(
+                scope = viewModelScope,
+                started = SharingStarted.WhileSubscribed(SHARE_STOP_TIMEOUT_MS),
+                initialValue = ThemeMode.fromWire(authStore.themeMode.value),
+            )
+
+    fun setThemeMode(mode: ThemeMode) {
+        // Persist null for SYSTEM so a "never set" row reads identically
+        // to an explicit "follow system" pick — keeps the wire/storage
+        // round-trip clean.
+        authStore.setThemeMode(if (mode == ThemeMode.SYSTEM) null else mode.wire)
+    }
+}