feat: add custom theme accent controls (R554)

[ryacub]

Ticket

• ID: R554
• Priority: P2
• Risk Tier: T2
• GitHub Issue: Closes R554: settings UX — add custom theme controls (accent picker + reset) #557

Objective

• Add custom theme accent controls (swatches + picker) for AppTheme.CUSTOM with safe persistence, clear reset behavior, and immediate theme refresh.

Scope

• Make AppTheme.CUSTOM visible/selectable in appearance theme options.
• Add custom accent controls shown only when custom theme is selected:
  • curated swatches
  • custom color picker dialog
  • reset accent action
• Persist accent via customThemeAccentSeed with opaque-alpha normalization.
• Ensure picker apply/cancel semantics and session handling avoid stale canceled state on reopen.
• Add test coverage for theme visibility, normalization, picker semantics, reset, flow persistence, and integration-style Android UI flow.

Non-goals

• No third-party color picker dependency.
• No docs/migration scope expansion beyond PR notes.

Files Changed

• app/src/main/java/eu/kanade/domain/ui/model/AppTheme.kt
• app/src/main/java/eu/kanade/presentation/more/settings/screen/SettingsAppearanceScreen.kt
• app/src/main/java/eu/kanade/presentation/more/settings/widget/AppThemePreferenceWidget.kt
• app/src/main/java/eu/kanade/presentation/more/settings/widget/CustomThemeAccentPreferenceWidget.kt
• i18n/src/commonMain/moko-resources/base/strings.xml
• app/src/test/java/eu/kanade/domain/ui/UiPreferencesTest.kt
• app/src/test/java/eu/kanade/presentation/more/settings/widget/AppThemePreferenceWidgetTest.kt
• app/src/test/java/eu/kanade/presentation/more/settings/widget/CustomThemeAccentPreferenceWidgetTest.kt
• app/src/test/java/eu/kanade/presentation/more/settings/screen/SettingsAppearanceScreenThemeFlowTest.kt
• app/src/androidTest/java/eu/kanade/presentation/more/settings/widget/CustomThemeAccentPreferenceWidgetAndroidTest.kt
• app/src/androidTest/java/eu/kanade/presentation/more/settings/widget/ThemeAppearanceFlowAndroidTest.kt
• app/build.gradle.kts

Verification

• Commands run:
  • ./gradlew spotlessCheck
  • ./gradlew :app:compileDebugKotlin
  • ./gradlew :app:testDebugUnitTest --tests "eu.kanade.presentation.more.settings.widget.AppThemePreferenceWidgetTest" --tests "eu.kanade.presentation.more.settings.widget.CustomThemeAccentPreferenceWidgetTest" --tests "eu.kanade.domain.ui.UiPreferencesTest" --tests "eu.kanade.presentation.more.settings.screen.SettingsAppearanceScreenThemeFlowTest"
  • ./gradlew :app:compileDebugAndroidTestKotlin
• Results:
  • All listed commands pass locally.
• Not tested:
  • Instrumentation execution on emulator/device (connectedDebugAndroidTest) was not run in this environment.

Risk

• Primary risk is UI behavior drift in appearance settings; mitigated by unit tests + Android test coverage for theme section flow and picker semantics.

SLO Impact

• No expected runtime SLO impact.

Rollback

• Revert commit df549a05e to restore previous appearance-settings behavior and remove custom accent controls.

Release Notes

• Added custom theme accent controls in Appearance settings with swatches, a custom color picker, and reset behavior.

Checklist

• Naming conventions followed (use "Rayniyomi" in user-facing text, see naming conventions)
• Branch rebased on latest main and verification re-run (see rebase policy)
• New coroutine scopes have documented owner and cancellation path
• No new runBlocking on UI/main thread paths

Definition of Done (DoD)

• Acceptance criteria met and non-goals respected
• Verification matrix completed (Risk Tier: T2)
• Self-review completed
• Rebased on latest main and revalidated (mandatory for merge)
• Sprint board updated (branch, commit, checks, PR link)
• Release notes drafted (if applicable)