fix: strip pubky prefix from truncated contact keys

[catch-21]

Truncated pubkys shown for contacts without a resolved profile read like pubky…poxyo. Every truncated pubky now goes through the single existing helper, which strips the prefix first and shows xxxx...xxxx.

Description

• PubkyPublicKeyFormat.display (app/src/main/java/to/bitkit/models/PubkyPublicKeyFormat.kt): also strips a pk: prefix (in addition to pubky) before taking the first 4 and last 4 characters; lengths are unchanged from what the Contacts screens already used.
• PubkyProfile.placeholder / PubkyProfile.forDisplay (app/src/main/java/to/bitkit/models/PubkyProfile.kt): the fallback name was publicKey.ellipsisMiddle(11), which produced pubky…poxyo. Both now use PubkyPublicKeyFormat.display. This is the value shown in every payment surface that falls back to a placeholder contact: SendSheet (contact payment preparing), SendConfirmScreen (contact payment profile), PaymentRequestsScreen, IncomingPaymentRequestDetailsScreen, CreatePaymentRequestScreen, SubscriptionsScreen, CreateSubscriptionScreen, ActivityDetailScreen, ContactActivityViewModel, ContactDetailViewModel, ContactsViewModel, PubkyChoiceViewModel, AppViewModel and PubkyRepo follow lists.
• PubkyAuthApprovalSheet.kt: dropped the local pubkyAuthDisplayPublicKey duplicate and used PubkyProfile.truncatedPublicKey for the profile card and success text.
• Full publicKey values used for copy, share, payments and storage are untouched; only the derived display string changed.
• changelog.d/next/1456.fixed.md: changelog fragment.

Out of Scope

• PubkyPublicKeyFormat.redacted: log-only redaction keeps its pubky3r…k8yw5xg form.
• ProfileScreen.kt / ContactDetailScreen.kt previews: literal preview publicKey = "pk8e3qm5...gxag" strings are preview fixtures, not display truncation.

Design

N/A — no design available.

Preview

N/A — could not run the app in this environment (no GitHub Packages credentials for com.synonym:* artifacts).

QA Notes

Journeys

N/A — not drivable; see Manual Tests.

Manual Tests

• Send to a Paykit contact that has no resolved profile (or pay a payment request from an unknown counterparty) → Send confirm / payment request sheets show the key as xxxx...xxxx without a pubky prefix — requires a second Paykit identity, not in Capabilities
• Approve a pubkyauth:// request → the profile card and success text show the key as xxxx...xxxx — requires a pubky auth relay, not in Capabilities

Automated Checks

• updated PubkyPublicKeyFormatTest.kt — display strips pubky and pk: prefixes, shortens unprefixed keys, and leaves short values unchanged
• removed PubkyAuthApprovalViewModelTest.kt — dropped the test for the deleted pubkyAuthDisplayPublicKey helper; coverage lives in PubkyPublicKeyFormatTest.kt
• ran: nothing — Gradle could not resolve com.synonym:ldk-node-android from GitHub Packages in the agent environment (GPR_USER/GPR_TOKEN not available), so unit tests and detekt rely on CI