feat: Unix socket IPC for send while sync is running

[vlassance]

Summary

• Start a Unix socket server at <store>/send.sock when sync --follow is running
• send text and send file try the socket first, falling back to direct connection
• Supports text messages and file uploads (image/video/audio/document) with MIME detection
• Socket uses JSON-over-newline protocol with 30s timeout (2min for file uploads)
• Socket file is created with 0600 permissions and cleaned up on shutdown

Problem

wacli send cannot work while sync --follow is running because both commands need the store. This makes it impossible to send messages programmatically while maintaining a sync session (see #6).

Solution

sync --follow now starts a Unix socket server that accepts send requests from other wacli processes. The send commands check for the socket first and route through the running sync daemon, falling back to a direct connection if sync is not running.

How it works

1. sync --follow starts a socket server at ~/.wacli/send.sock
2. send text / send file check if the socket exists and try IPC first
3. If IPC succeeds, the message is sent through the running sync daemon's connection
4. If IPC fails or socket doesn't exist, falls back to direct mode

Protocol

Each request/response is a single JSON line terminated by \n:

{"to": "+1234567890", "message": "hello"}
{"to": "+1234567890", "type": "file", "file_path": "/tmp/photo.jpg", "caption": "check this out"}

Testing

• go test ./... — all tests pass
• go build ./... — builds cleanly

Closes #6