Implement encrypted messaging protocol using shielded transaction memos

[Copilot]

Adds infrastructure for end-to-end encrypted messaging via Zcash shielded transaction memos. Messages are encrypted with the recipient's viewing key and can only be decrypted by the intended recipient.

Core Protocol (core/src/memo.rs)

• Memo format: 512-byte structure with version, type (text/ack/fragment), timestamp, nonce, and UTF-8 payload
• Fragmentation: Messages >498 bytes automatically split across multiple memos with reassembly support
• Deduplication: Nonce-based duplicate detection

// Encode a message into a memo
let memo = encode_message_memo("Hello, Zcash!", timestamp, nonce)?;

// Decode received memo
let message = decode_message_memo(&memo_bytes)?;

// Handle long messages automatically
let fragments = encode_message_fragments(&long_text, timestamp, nonce)?;
let reassembled = reassemble_fragments(&decoded_fragments)?;

WASM Bindings (wasm-module/src/lib.rs)

Exposed 5 JavaScript functions:

• encode_message_memo(text, timestamp, nonce)
• encode_message_fragments(text, timestamp, nonce)
• decode_message_memo(memo_hex)
• reassemble_message_fragments(fragments_json)
• generate_message_nonce()

Storage & Sync (frontend/js)

• localStorage-based message database with conversation grouping, status tracking, and fragment management
• Sync service extracts and parses memos from notes during wallet sync, automatically reassembles fragments

Limitations

Sending messages requires shielded transaction support (Sapling/Orchard), which is not yet implemented. The infrastructure is ready for when shielded sending is added. Currently supports viewing received messages only.

Test Coverage

• 12 core Rust tests covering encoding, decoding, fragmentation, and edge cases
• 3 WASM integration tests
• All clippy checks passing with -D warnings

Original prompt

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This section details on the original issue you should resolve

<issue_title>Messaging</issue_title>
<issue_description>Implement an encrypted messaging system for LeakIX/zcash-web-wallet using shielded transaction memos.

Repository Context

• Repo: https://github.com/LeakIX/zcash-web-wallet
• Stack: Rust core + WASM bindings, Bootstrap + vanilla JS frontend
• Existing: BIP39 wallet generation, Orchard shielded pool support, transaction viewer
• Architecture: All crypto in WASM, keys never leave browser

Background

Zcash shielded transactions include a 512-byte memo field that is encrypted along with the note. Only the recipient (holder of the incoming viewing key) can decrypt and read memos. This provides native E2E encryption for messages without additional key exchange.

Requirements

Phase 1: Memo Protocol (Rust/WASM)

1. Define memo format in core/src/memo.rs:
   [0] version (0x01)
   [1] type (0x00=text, 0x01=ack, 0x02=fragment)
   [2-5] timestamp (u32, unix epoch)
   [6-9] nonce (u32, for dedup/ordering)
   [10-13] fragment info (if type=0x02): total_fragments(u16) + index(u16)
   [14-511] payload (UTF-8 text, null-padded)

2. Implement in WASM module (wasm-module/src/lib.rs):

   • encode_message_memo(text: &str, timestamp: u32) -> Vec<u8>
   • decode_message_memo(memo: &[u8]) -> Result<Message, MemoError>
   • create_message_transaction(recipient_addr: &str, memo: Vec<u8>, amount: u64) -> Result<Transaction, Error>
   • Fragment long messages (>498 bytes) across multiple memos

3. Extend existing transaction builder to accept memo parameter

Phase 2: Message Storage (IndexedDB)

1. Schema (frontend/js/storage/messages.js):

{
  id: string,           // txid + output_index
  conversation: string, // recipient z-addr (normalized)
  direction: 'sent' | 'received',
  content: string,
  timestamp: number,
  blockHeight: number | null,
  status: 'pending' | 'confirmed',
  fragmentGroup: string | null
}
Conversations index: group by z-addr, sort by latest message
Phase 3: Sync Service
Message scanner (frontend/js/services/message-sync.js):
On wallet sync, extract memos from incoming notes
Parse memo format, filter message types
Reassemble fragmented messages
Store in IndexedDB
Track sent messages via pending tx pool
Real-time updates:
Poll for new blocks (or WebSocket if available)
Update message status pending → confirmed
Phase 4: UI Components
Conversation list (frontend/js/components/conversations.js):
List of z-addrs with last message preview
Unread indicator
Sort by recency
Chat view (frontend/js/components/chat.js):
Message bubbles (sent/received styling)
Timestamp display
Pending indicator (spinner → checkmark)
Input field with character counter (max ~498 chars or show fragment warning)
Send button (triggers transaction creation)
New conversation: address input with validation
Phase 5: Transaction Integration
Send message flow:
User enters text → encode memo → create shielded tx with dust amount
Sign and broadcast via existing tx flow
Add to pending messages
Update UI optimistically
Cost display: Show estimated fee (~0.0001 ZEC per message)
Technical Constraints
Memo size: 512 bytes total, ~498 usable after header
Minimum tx amount: dust threshold (use smallest valid amount)
Confirmation time: ~75 seconds per block
No delivery receipts (fire-and-forget by design)
Messages are immutable once on-chain
Files to Create/Modify
New files:
core/src/memo.rs - Memo encoding/decoding
wasm-module/src/memo.rs - WASM bindings for memo ops
frontend/js/storage/messages.js - IndexedDB wrapper
frontend/js/services/message-sync.js - Blockchain message scanner
frontend/js/components/conversations.js - Conversation list UI
frontend/js/components/chat.js - Chat interface
Modified:
core/src/lib.rs - Export memo module
wasm-module/src/lib.rs - Expose memo functions
frontend/index.html - Add messaging tab/section
frontend/js/app.js - Wire up messaging components
Deliverables (in order)
Memo protocol implementation (Rust)
WASM bindings + tests
IndexedDB storage layer
Message sync service
UI components
Integration with existing wallet flow
Start with the Rust memo protocol and WASM bindings.</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Messaging #121

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