Add delivery diagnostics, event previews and settings transfer

[andre487]

Add tools for diagnosing delivery, inspecting event contents before an explicit send, and transferring saved settings between devices.

• Overview checks explain connection, source permissions, listener connectivity, selected apps, network, background restrictions and delivery failures, with links to corrective actions.
• Connection previews test events; Sources previews current notifications from selected apps using the capture text/image pipeline. Opening a preview does not enqueue it. Explicit test sends work during Pause without resuming capture or ordinary queued delivery; the exception persists in WorkManager input for automatic retries. Real notification previews remain blocked during Pause.
• Settings use Android's document picker for versioned JSON export/import. The Bearer token is included by default with a prominent opt-out and plaintext-file disclosure. Import validates the entire file, shows confirmation and pauses forwarding. Omitting the token clears the previous token, including unreadable ciphertext. Installation identity, permissions, queue, history and update preferences are preserved.
• Events include string channel_id: the Android notification channel ID, sms for SMS and test for tests. The additive field keeps schema version 1; older queued bodies are unchanged.

Update EN/RU UI strings, store descriptions, webhook documentation and privacy details. Also fix an existing update-notification test race by waiting for both the saved preference and notification removal.

Validation:

• All 154 JVM/Robolectric tests and debug/release lint passed.
• Debug APK built, installed and launched on the API 35 emulator; checked the export opt-out and two-image gallery preview without sending to a receiver.
• Regression checks cover import validation/confirmation, token handling, channel persistence across retries, preview privacy, image-file retention and paused manual-test scheduling.
• Store description lengths and git diff --check passed.