feat(ui): Settings theme picker dialog (#87)

[ilmoniemi]

What

Wires the Settings → Appearance → Theme row to an M3 single-choice AlertDialog ("System default" / "Light" / "Dark") and hoists the read + write of ThemeMode out of the Settings route composable into a new SettingsViewModel.

• SettingsViewModel — exposes themeMode: StateFlow<ThemeMode> via stateIn(viewModelScope, WhileSubscribed(5_000L), SYSTEM) (mirrors ChannelListViewModel shape) and onSelectTheme(mode) that writes through AppPreferences.setThemeMode(...).
• ThemePickerDialog — new internal composable in ui/settings/. M3 AlertDialog with a selectableGroup() column of three radio rows tracking local pending state (remember(selected)), OK/Cancel buttons; confirm → onConfirm(pending), cancel/dismiss → onDismiss().
• SettingsScreen — new signature (themeMode, onSelectTheme, onBack, onOpenLicense, …); old themeMode = SYSTEM default deleted. Theme row onClick flips a local showThemeDialog open-state. ThemeMode.label() extension promoted from private to internal so the dialog composable reuses it — single source of truth for the three label strings.
• MainActivity — Settings route now uses koinViewModel<SettingsViewModel>() + vm.themeMode.collectAsStateWithLifecycle(), replacing direct AppPreferences injection. Root-level appPreferences.themeMode collector that drives PyrycodeMobileTheme(darkTheme = …) is untouched, so the persisted write fans out to both surfaces and the app theme re-renders without a process restart.
• AppModule — one-line viewModel { SettingsViewModel(get()) } Koin registration.
• strings.xml — adds settings_theme_dialog_title ("Theme"). Radio labels reuse ThemeMode.label().

Issue

Closes #87

Testing

• ./gradlew test — pass (new SettingsViewModelTest covers initial state, mirror-of-persisted-value, persistence of each of LIGHT/DARK/SYSTEM, and re-emission after onSelectTheme).
• ./gradlew lint — pass.
• ./gradlew assembleDebug — pass.
• ./gradlew connectedAndroidTest — not run (no device/emulator wired in this worktree). SettingsScreenTest was updated to pass the new params (compile-only verified).

Architecture compliance

Follows the spec at docs/specs/architecture/87-settings-theme-picker-dialog.md verbatim: SettingsViewModel shape mirrors ChannelListViewModel (stateIn + WhileSubscribed(5_000L)); dialog uses OK/Cancel (decided in spec § Design); ThemeMode.label() promoted to internal (decided in spec § Label helper); dialog open-state is local to the screen composable; pending keyed by selected so a reopen picks up an updated persisted value (decided in spec § State + concurrency model); MainActivity root-level collector left intact (decided in spec § Files to read first).

🤖 Generated with Claude Code

[ilmoniemi]

Code Review: #87

Decision: PASS

Findings

No MUST FIX / SHOULD FIX findings.

• [NIT] ThemePickerDialog.kt:43,48 — using android.R.string.ok / android.R.string.cancel for the dialog buttons works (localized for free) but diverges slightly from the rest of the codebase, which keeps button strings in app/src/main/res/values/strings.xml (e.g. promote_dialog_confirm, promote_dialog_cancel). Not a blocker — flagging only because future localisation passes may want consistency. The spec didn't prescribe which path to take.

Verified

• SettingsViewModel follows ChannelListViewModel's canonical shape exactly — stateIn(viewModelScope, WhileSubscribed(5_000L), initial) for the read flow, viewModelScope.launch for the write. No sealed SettingsState/SettingsEvent (correctly deferred per spec).
• ThemePickerDialog uses the correct accessibility pattern: Modifier.selectableGroup() on the column + Modifier.selectable(..., role = Role.RadioButton, onClick = …) on each row with RadioButton(onClick = null) — avoids the dual-click-handler trap. MaterialTheme.typography.bodyLarge used; no hardcoded colors / TextStyle() literals anywhere in the diff.
• remember(selected) keying on pending is correct — if the persisted value changes while the dialog is closed and the parent recomposes with a new selected, the next open picks it up. Confirm path calls onSelectTheme(pending) → VM writes; Cancel/dismiss discards pending via composition exit. Matches AC #2.
• ThemeMode.label() promoted from private to internal (single source of truth, shared between row subtitle and dialog labels). Not moved into data/preferences/ — correct, these are UI strings and data/ is the CMP walk-back boundary.
• MainActivity.kt:54-62 (root-level theme collection) untouched. The new VM adds a second subscriber to the same DataStore flow; one write fans out to both surfaces, which is what AC #3 depends on. Verified koinInject import retained (Scanner route + root theme still use it).
• SettingsViewModelTest reuses the AppPreferencesTest rig (real PreferenceDataStoreFactory + TemporaryFolder + real AppPreferences) — exercises the round-trip end-to-end. All six scenarios from the spec covered, including the flow re-emit gate.
• SettingsScreenTest (instrumented) updated to pass the new required params; both @Preview composables updated. No stale defaults.
• ./gradlew :app:testDebugUnitTest and ./gradlew :app:lintDebug clean locally.

Summary

Slice is small, focused, and faithfully tracks the spec. No code-level findings, no acceptance-criteria gaps. Approved.