feat(settings): wire "Use wallpaper colors" switch to AppPreferences

[ilmoniemi]

Summary

• Replaces the local-only materialYou placeholder row in Settings → Appearance with a VM-backed, SDK-gated switch row bound to AppPreferences.useWallpaperColors.
• Extends SettingsViewModel with useWallpaperColors: StateFlow<Boolean> and onToggleUseWallpaperColors(enabled), mirroring the existing themeMode / onSelectTheme pair line-for-line.
• On Android 12+: switch is enabled, reflects persisted state, and toggling fans out via the existing composition-root collector wired in feat(theme): persist useWallpaperColors preference + dynamic ColorScheme branching #88 — live theme update, no restart.
• On Android < 12: switch renders disabled with helper text "Requires Android 12 or newer" (structurally unreachable today since Min SDK 33 > S, but implemented per AC feat(data): add ConversationRepository interface #3).
• Label diverges from Figma node 17-2 per the spec's documented decision: ticket body says "Use wallpaper colors"; Figma still shows the older "Use Material You dynamic color" copy.

Issue

Closes #89

Testing

• ./gradlew :app:testDebugUnitTest — pass. Three new tests in SettingsViewModelTest covering: initial state when no stored value, initial state mirrors persisted true, and a single round-trip test asserting both persistence and flow re-emission on toggle (true then false).
• ./gradlew :app:lintDebug — pass.
• ./gradlew :app:assembleDebug — pass.
• ./gradlew connectedAndroidTest — not run (no device/emulator connected in this worktree).
• ./gradlew spotlessCheck — pre-existing failures in ThemePickerDialog.kt and Theme.kt (both unchanged on this branch and failing on main already); my five modified files are clean. Out of scope per CLAUDE.md ("don't refactor adjacent code").

Architecture compliance

Implements the spec at docs/specs/architecture/89-settings-use-wallpaper-colors-switch-row.md verbatim:

• VM gains the documented StateFlow + event method with WhileSubscribed(STOP_TIMEOUT_MILLIS) and initialValue = false.
• SettingsScreen signature adds useWallpaperColors: Boolean and onToggleUseWallpaperColors: (Boolean) -> Unit in the order specified (state values, event callbacks, nav callbacks, modifier).
• SDK gate computed once at composition build with Build.VERSION.SDK_INT >= Build.VERSION_CODES.S — no remember.
• MainActivity settings route adds the second collectAsStateWithLifecycle() and wires both new params; the root-level dynamicColor collector at lines 62-63 is untouched.
• New strings extracted to strings.xml; no fake AppPreferences introduced (real DataStore via TemporaryFolder).
• No sealed SettingsState/SettingsEvent — deferred per the spec's premature-abstraction note.

🤖 Generated with Claude Code

[ilmoniemi]

Code Review: #89

Decision: PASS

Findings

No MUST FIX or SHOULD FIX issues.

• VM additions in SettingsViewModel.kt:22-27, 33-35 mirror the existing themeMode / onSelectTheme pair line-for-line, including reuse of STOP_TIMEOUT_MILLIS and initialValue = false (matches AppPreferences.useWallpaperColors's default-when-absent, so no one-frame flash).
• Row replacement in SettingsScreen.kt:120-135 follows the spec exactly: SDK gate evaluated once at composition build (val dynamicColorSupported = Build.VERSION.SDK_INT >= Build.VERSION_CODES.S at line 59 — no unnecessary remember), enabled = dynamicColorSupported on the Switch gates taps via M3 idiom rather than nulling the callback, helper text routed through the existing supporting slot on the disabled-pre-S branch only.
• The deleted var materialYou by remember { mutableStateOf(true) } removes the placeholder cleanly; no orphaned imports left behind. MainActivity.kt:200-213 wires the new state + event correctly without disturbing the root-level useWallpaperColors collector at lines 62-63 (the load-bearing piece for AC feat(data): add Conversation, Session, Message data classes #2's live re-theme).
• Strings extracted to R.string.settings_use_wallpaper_colors and R.string.settings_use_wallpaper_colors_unsupported; the other inline-string switch rows on the screen are correctly left out of scope.
• Tests in SettingsViewModelTest.kt:120-160 cover the three scenarios from the spec exactly (initial-false-when-absent, mirrors-persisted-true, toggle persists + flow re-emits), reusing the existing TemporaryFolder rig and the real AppPreferences. The combined persistence + re-emission test is the right call — splitting would just be noise.
• Both @Preview composables updated with useWallpaperColors = false and onToggleUseWallpaperColors = {}. No third preview for the disabled-pre-S state (correct — Min SDK 33 > S, the branch is structurally unreachable today).
• Figma fidelity (node 17-2): row layout, height, surface container, trailing Switch all match. Label divergence ("Use wallpaper colors" vs. Figma's "Use Material You dynamic color") is explicitly justified in the spec's Context section — ticket body is authoritative.
• M3 token usage clean: ListItemDefaults.colors(containerColor = MaterialTheme.colorScheme.surface) via the existing SettingsRow; no hardcoded colors, typography, or shape values in the diff.

Summary

Implementation matches the architecture spec exactly. The row is properly VM-driven, the SDK gate uses the M3 enabled idiom (cleaner than null-ing onCheckedChange), and the tests exercise the full DataStore round-trip rather than mocking the layer under test. The independent VM-side collector pairs naturally with #88's composition-root collector — one DataStore write fans out to both subscribers, giving the live re-theme behavior AC #2 calls for. Ready to merge.