Port iOS geohash DM, payload cap, and panic confirm fixes

Ports three security/privacy fixes from bitchat iOS and pins a fourth that Android already handles.

Changes

Geohash DMs no longer carry the mesh peer ID (jackjackbits/bitchat#1718) The recipient-less encoders used for geohash DMs and their acks wrote our mesh peer ID into the embedded packet's sender field. That ID is the stable Noise fingerprint prefix, so anyone who DMs a geohash identity (and gets the automatic DELIVERED ack) could link that persona to the device. Each envelope now gets fresh random bytes. No iOS or Android receiver reads this field; favorites (recipient-addressed) envelopes are unchanged.

Per-type payload caps at decode (decoder part of jackjackbits/bitchat#1719) New PacketPayloadLimits with the same per-type caps as iOS (public message 128 KiB, request sync 8 KiB, announce/leave/handshake/voice 4 KiB, fragment 1,280 B, unknown types 128 KiB). FILE_TRANSFER and NOISE_ENCRYPTED keep the existing MAX_PAYLOAD_LENGTH. Enforced on compressed and uncompressed payloads before allocation. The 50,000:1 compression-ratio guard becomes DEFLATE's 1032:1 bound. Not ported here: the iOS reassembly bound by claimed type and byte budgets for sync stores.

Panic wipe asks first (jackjackbits/bitchat#1588) Triple-tapping the logo now opens a "Wipe all data?" dialog instead of wiping immediately. A completed wipe posts "all data wiped — new identity created"; a wipe whose conversation-database erase fails now says so in the chat instead of failing silently. New strings are English-only (tools:ignore="MissingTranslation"), following recent precedent.

Ack only after the block check Already correct on Android (the block check runs right after decryption). Added a regression test.

Risk

• Payload caps are receive-side only and sized at ≥2× the largest payload current iOS/Android encoders produce, but an oversized legitimate packet of a capped type would now be dropped at decode.
• The sender-ID change touches the geohash DM wire content; receivers key these conversations on the Nostr pubkey, so interop should be unaffected.
• Panic flow gains one extra tap.

Testing

• ./gradlew testDebugUnitTest lintDebug passes (app 659 tests, Wear 231 tests); no new lint findings in changed code.
• New tests: PacketPayloadLimitsTest, NostrEmbeddedBitChatTest, blocked-sender case in NostrDirectMessageHandlerTest; existing BinaryProtocolTest fixtures updated for the new bounds.
• Not yet done: Mesh Lab validation on physical devices (iOS↔Android geohash DMs, large public messages and file transfers), and before/after screenshots of the panic confirmation dialog.