feat: add contact payment ui

[ben-kaufman]

Description

This PR:

1. Adds Send → Contact selection so saved contacts can be chosen from the send sheet.
2. Adds contact attribution UI on Activity Detail, including assigning and detaching a contact.
3. Shows contact avatars in the main and full activity lists while keeping the contact-specific activity list on the existing arrow style.
4. Adds Payment Preference settings for Lightning/on-chain methods and public/private contact sharing.
5. Wires public/private Paykit endpoint publishing to the selected payment preferences.

Preview

QA Notes

Manual Tests

• 1. Wallet → Send → Contact: saved contacts appear and tapping one opens the normal send flow for that contact.
• 2. Activity → Activity Detail → Assign: contact picker opens, selecting a contact shows the assigned contact section, Detach removes it.
• 3. Activity list → contact-attributed payment: main and full activity lists show the contact avatar instead of the arrow icon.
• 4. Contact Detail → Activity: contact-specific activity rows still show the arrow icon and the contact wording.
• 5. Settings → General → Payment Preference: Lightning/on-chain toggles cannot both be disabled; public/private contact toggles sync Paykit sharing state.

Automated Checks

• Local checks passed: ./gradlew compileDevDebugKotlin, ./gradlew testDevDebugUnitTest, ./gradlew detekt, git diff --check.
• E2E build passed: E2E=true E2E_BACKEND=network ./gradlew assembleDevDebug --no-daemon --stacktrace.
• Android emulator proof captured for contact detail, Payment Preference, and Send → Contact. A two-device profile/contact smoke flow was partially exercised; Pubky staging lookup needed a manual retry, so full automated two-device payment E2E should be rerun in CI or a stable staging session.

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