feat(updates): add selectable release channels

[veryCrunchy]

What changed

• add an accessible Alpha and Nightly selector for direct APK installations
• represent Beta and Stable as disabled future-compatible channels
• persist stable lowercase channel identifiers and migrate older stored values
• bind metadata checks and installation to the currently selected channel
• invalidate stale check results when the channel changes
• reject unavailable channels, cross-channel metadata, and Android version-code downgrades
• classify Android 13+ local and downloaded package sources as direct APK installs while keeping known and unknown stores conservative
• keep package, SDK, asset-size, checksum, release URL, signer, and signer-lineage validation in the install path

User impact

People who install Nextcloud Native directly can choose curated Alpha releases or the latest signed Nightly build without reinstalling or relying on an app store. Store-managed installations continue to follow their store and do not expose the selector.

Validation

• 7 update-channel presentation and policy tests pass
• 5 release metadata and channel-binding tests pass
• 13 Android project-content and installer-source tests pass
• repository hygiene, release contracts, ASCII hygiene, and diff checks pass
• Android and shared tests run with JDK 21 and one build worker

Closes #228

[chatgpt-codex-connector]

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