feat(settings): add in-app theme mode override (System/Light/Dark)

[darky]

#1074

[greptile-apps]

Greptile Summary

The PR adds a persisted System/Light/Dark appearance override and applies the effective mode across the main UI, theme editor, previews, incoming-call UI, and system bars.

• Adds ThemeMode persistence and Settings UI controls.
• Propagates the effective mode through application and incoming-call themes.
• Keeps incoming-call controls available while loading the persisted preference.
• Synchronizes theme-editor previews and status/navigation-bar icon appearance.
• Adds repository, ViewModel, mode-resolution, and incoming-call tests.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
app/src/main/java/network/columba/app/IncomingCallActivity.kt	Collects theme mode with an immediate System fallback and synchronizes incoming-call colors and system-bar icon appearance without delaying call controls.
app/src/main/java/network/columba/app/MainActivity.kt	Resolves the effective app mode for the root theme and supplies it to both theme-editor destinations.
app/src/main/java/network/columba/app/ui/screens/ThemeEditorScreen.kt	Initializes and resynchronizes the preview variant from the effective application mode.
app/src/main/java/network/columba/app/ui/theme/Theme.kt	Synchronizes both status-bar and navigation-bar icon brightness with the active color scheme.
app/src/main/java/network/columba/app/repository/SettingsRepository.kt	Adds typed DataStore persistence for the System/Light/Dark mode preference.
app/src/main/java/network/columba/app/viewmodel/SettingsViewModel.kt	Incorporates persisted theme mode into settings state and exposes the update action.
app/src/main/java/network/columba/app/ui/screens/settings/cards/ThemeSelectionCard.kt	Adds localized mode-selection chips and renders color previews using the effective mode.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    UI[Theme mode chips] --> VM[SettingsViewModel]
    VM --> Repo[SettingsRepository]
    Repo --> DS[Preferences DataStore]
    DS --> State[ThemeMode Flow]
    State --> Main[MainActivity / ColumbaTheme]
    State --> Call[IncomingCallActivity]
    Main --> Editor[Theme editor and preview]
    Main --> Bars[Status and navigation bars]
    Call --> Bars

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_{Reviews (7): Last reviewed commit: "fix(calls): render incoming controls imm..." | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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